BUG: Respect color argument in bar plot
This affect Series-like barplots with user-defined colors. We were always
treating them as dataframe-like, with one color per column.

closes https://github.com/pandas-dev/pandas/issues/16822

This should go into 0.21.0, but if I have the time I'd like a put a more elegant solution in place.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17360?src=pr&el=h1) Report
> Merging [#17360](https://codecov.io/gh/pandas-dev/pandas/pull/17360?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/f5cfdbb1f4b715819aceac4b00cf18ba5f467f85?src=pr&el=desc) will **decrease** coverage by `0.25%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/17360/graphs/tree.svg?height=150&width=650&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/17360?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17360      +/-   ##
==========================================
- Coverage   91.25%   90.99%   -0.26%     
==========================================
  Files         163      162       -1     
  Lines       49614    49570      -44     
==========================================
- Hits        45274    45107     -167     
- Misses       4340     4463     +123
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.77% <100%> (-0.25%)` | :arrow_down: |
| #single | `40.24% <0%> (-0.02%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17360?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/plotting/\_core.py](https://codecov.io/gh/pandas-dev/pandas/pull/17360?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb3JlLnB5) | `82.72% <100%> (+0.02%)` | :arrow_up: |
| [pandas/io/s3.py](https://codecov.io/gh/pandas-dev/pandas/pull/17360?src=pr&el=tree#diff-cGFuZGFzL2lvL3MzLnB5) | `0% <0%> (-85%)` | :arrow_down: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17360?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/tools/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/17360?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvdG9vbHMvZGF0ZXRpbWVzLnB5) | `67.03% <0%> (-18.14%)` | :arrow_down: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/17360?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `63.23% <0%> (-1.82%)` | :arrow_down: |
| [pandas/core/reshape/pivot.py](https://codecov.io/gh/pandas-dev/pandas/pull/17360?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS9waXZvdC5weQ==) | `95.36% <0%> (-1%)` | :arrow_down: |
| [pandas/core/indexes/interval.py](https://codecov.io/gh/pandas-dev/pandas/pull/17360?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9pbnRlcnZhbC5weQ==) | `92.61% <0%> (-0.96%)` | :arrow_down: |
| [pandas/io/common.py](https://codecov.io/gh/pandas-dev/pandas/pull/17360?src=pr&el=tree#diff-cGFuZGFzL2lvL2NvbW1vbi5weQ==) | `67.79% <0%> (-0.85%)` | :arrow_down: |
| [pandas/core/indexes/timedeltas.py](https://codecov.io/gh/pandas-dev/pandas/pull/17360?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy90aW1lZGVsdGFzLnB5) | `90.58% <0%> (-0.61%)` | :arrow_down: |
| [pandas/core/common.py](https://codecov.io/gh/pandas-dev/pandas/pull/17360?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tbW9uLnB5) | `91.44% <0%> (-0.54%)` | :arrow_down: |
| ... and [35 more](https://codecov.io/gh/pandas-dev/pandas/pull/17360?src=pr&el=tree-more) | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17360?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/17360?src=pr&el=footer). Last update [f5cfdbb...e02d8fd](https://codecov.io/gh/pandas-dev/pandas/pull/17360?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/17360?src=pr&el=h1) Report
> Merging [#17360](https://codecov.io/gh/pandas-dev/pandas/pull/17360?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/f5cfdbb1f4b715819aceac4b00cf18ba5f467f85?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/17360/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/17360?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17360      +/-   ##
==========================================
- Coverage   91.25%   91.23%   -0.02%     
==========================================
  Files         163      163              
  Lines       49614    49617       +3     
==========================================
- Hits        45274    45268       -6     
- Misses       4340     4349       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.02% <100%> (ø)` | :arrow_up: |
| #single | `40.19% <0%> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17360?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/plotting/\_core.py](https://codecov.io/gh/pandas-dev/pandas/pull/17360?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb3JlLnB5) | `82.73% <100%> (+0.03%)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17360?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17360?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.77% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17360?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/17360?src=pr&el=footer). Last update [f5cfdbb...e02d8fd](https://codecov.io/gh/pandas-dev/pandas/pull/17360?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
