EHN: Add index parameter to to_json
- [ ] closes #17394
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Examples:
```
In [1]: df = pd.DataFrame([[1, 2, 3], [4, 5, 6]], columns=['a', 'b', 'c'])

In [2]: df.to_json(orient='split', index=True)
Out[2]: 
{"columns":["a","b","c"],"index":[0,1],"data":[[1,2,3],[4,5,6]]}

In [3]: df.to_json(orient='split', index=False)
Out[3]:
{"columns":["a","b","c"],"data":[[1,2,3],[4,5,6]]}

In [4]: df.to_json(orient='table', index=True)
Out[4]:
{"schema": {"fields":[{"name":"index","type":"integer"},{"name":"a","type":"integer"},{"name":"b","type":"integer"},{"name":"c","type":"integer"}],"primaryKey":["index"],"pandas_version":"0.20.0"}, "data": [{"index":0,"a":1,"b":2,"c":3},{"index":1,"a":4,"b":5,"c":6}]}

In [5]: df.to_json(orient='table', index=False)
Out[5]:
{"schema": {"fields":[{"name":"a","type":"integer"},{"name":"b","type":"integer"},{"name":"c","type":"integer"}],"pandas_version":"0.20.0"}, "data": [{"a":1,"b":2,"c":3},{"a":4,"b":5,"c":6}]}
```
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18591?src=pr&el=h1) Report
> Merging [#18591](https://codecov.io/gh/pandas-dev/pandas/pull/18591?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/d74ac70ffd44e9d00e6ffa8ecf7a1a88312e8065?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18591/graphs/tree.svg?height=150&width=650&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/18591?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18591      +/-   ##
==========================================
- Coverage   91.44%   91.43%   -0.02%     
==========================================
  Files         157      157              
  Lines       51379    51393      +14     
==========================================
+ Hits        46985    46992       +7     
- Misses       4394     4401       +7
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.3% <100%> (ø)` | :arrow_up: |
| #single | `40.56% <12.5%> (-0.12%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18591?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/18591/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `95.78% <ø> (ø)` | :arrow_up: |
| [pandas/io/json/json.py](https://codecov.io/gh/pandas-dev/pandas/pull/18591/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2pzb24vanNvbi5weQ==) | `91.78% <100%> (+0.03%)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/18591/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/18591/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.81% <0%> (-0.1%)` | :arrow_down: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/18591/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `81.8% <0%> (+0.19%)` | :arrow_up: |
| [pandas/io/json/table\_schema.py](https://codecov.io/gh/pandas-dev/pandas/pull/18591/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2pzb24vdGFibGVfc2NoZW1hLnB5) | `97.22% <0%> (+1.38%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18591?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18591?src=pr&el=footer). Last update [d74ac70...df61cee](https://codecov.io/gh/pandas-dev/pandas/pull/18591?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18591?src=pr&el=h1) Report
> Merging [#18591](https://codecov.io/gh/pandas-dev/pandas/pull/18591?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/d74ac70ffd44e9d00e6ffa8ecf7a1a88312e8065?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18591/graphs/tree.svg?token=eZ4WkYLtcO&width=650&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/18591?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18591      +/-   ##
==========================================
- Coverage   91.44%   91.43%   -0.02%     
==========================================
  Files         157      157              
  Lines       51379    51393      +14     
==========================================
+ Hits        46985    46992       +7     
- Misses       4394     4401       +7
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.3% <100%> (ø)` | :arrow_up: |
| #single | `40.56% <12.5%> (-0.12%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18591?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/18591/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `95.78% <ø> (ø)` | :arrow_up: |
| [pandas/io/json/json.py](https://codecov.io/gh/pandas-dev/pandas/pull/18591/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2pzb24vanNvbi5weQ==) | `91.78% <100%> (+0.03%)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/18591/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/18591/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.81% <0%> (-0.1%)` | :arrow_down: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/18591/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `81.8% <0%> (+0.19%)` | :arrow_up: |
| [pandas/io/json/table\_schema.py](https://codecov.io/gh/pandas-dev/pandas/pull/18591/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2pzb24vdGFibGVfc2NoZW1hLnB5) | `97.22% <0%> (+1.38%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18591?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18591?src=pr&el=footer). Last update [d74ac70...df61cee](https://codecov.io/gh/pandas-dev/pandas/pull/18591?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18591?src=pr&el=h1) Report
> Merging [#18591](https://codecov.io/gh/pandas-dev/pandas/pull/18591?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/d74ac70ffd44e9d00e6ffa8ecf7a1a88312e8065?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18591/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/18591?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18591      +/-   ##
==========================================
- Coverage   91.44%   91.43%   -0.02%     
==========================================
  Files         157      157              
  Lines       51379    51393      +14     
==========================================
+ Hits        46985    46992       +7     
- Misses       4394     4401       +7
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.3% <100%> (ø)` | :arrow_up: |
| #single | `40.56% <12.5%> (-0.12%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18591?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/18591/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `95.78% <ø> (ø)` | :arrow_up: |
| [pandas/io/json/json.py](https://codecov.io/gh/pandas-dev/pandas/pull/18591/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2pzb24vanNvbi5weQ==) | `91.78% <100%> (+0.03%)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/18591/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/18591/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.81% <0%> (-0.1%)` | :arrow_down: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/18591/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `81.8% <0%> (+0.19%)` | :arrow_up: |
| [pandas/io/json/table\_schema.py](https://codecov.io/gh/pandas-dev/pandas/pull/18591/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2pzb24vdGFibGVfc2NoZW1hLnB5) | `97.22% <0%> (+1.38%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18591?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18591?src=pr&el=footer). Last update [d74ac70...df61cee](https://codecov.io/gh/pandas-dev/pandas/pull/18591?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18591?src=pr&el=h1) Report
> Merging [#18591](https://codecov.io/gh/pandas-dev/pandas/pull/18591?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/34a8d36e45c1623d2e61671561cdbd5de36adac8?src=pr&el=desc) will **decrease** coverage by `0.04%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18591/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/18591?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18591      +/-   ##
==========================================
- Coverage    91.6%   91.56%   -0.05%     
==========================================
  Files         153      153              
  Lines       51273    51290      +17     
==========================================
- Hits        46970    46965       -5     
- Misses       4303     4325      +22
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.43% <100%> (-0.03%)` | :arrow_down: |
| #single | `40.7% <19.04%> (-0.09%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18591?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/18591/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `95.9% <ø> (ø)` | :arrow_up: |
| [pandas/io/json/json.py](https://codecov.io/gh/pandas-dev/pandas/pull/18591/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2pzb24vanNvbi5weQ==) | `92.08% <100%> (+0.33%)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/18591/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/18591/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `64.78% <0%> (-1.74%)` | :arrow_down: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/18591/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `81.82% <0%> (-0.2%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/18591/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.81% <0%> (-0.1%)` | :arrow_down: |
| [pandas/io/json/table\_schema.py](https://codecov.io/gh/pandas-dev/pandas/pull/18591/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2pzb24vdGFibGVfc2NoZW1hLnB5) | `97.22% <0%> (+1.38%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18591?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18591?src=pr&el=footer). Last update [34a8d36...b204d09](https://codecov.io/gh/pandas-dev/pandas/pull/18591?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 looks good. rebase and ping on green. @jreback thanks! Green now. lgtm. @jorisvandenbossche merge when satisfied. @reidy-p Thanks!