Fix order parameters and add decimal to to_string
- [x] closes #23612
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry
Add an extra argument:`decimal` to `pandas.DataFrame.to_string`. 
Hello @thoo! Thanks for updating the PR.

 - There are no PEP8 issues in the file [`pandas/core/frame.py`](https://github.com/thoo/pandas/blob/87297cdcc934e82b73061e0e5d776c54a39ab58b/pandas/core/frame.py) !

 - There are no PEP8 issues in the file [`pandas/io/formats/format.py`](https://github.com/thoo/pandas/blob/87297cdcc934e82b73061e0e5d776c54a39ab58b/pandas/io/formats/format.py) !

 - There are no PEP8 issues in the file [`pandas/tests/io/formats/test_format.py`](https://github.com/thoo/pandas/blob/87297cdcc934e82b73061e0e5d776c54a39ab58b/pandas/tests/io/formats/test_format.py) !



##### Comment last updated on November 11, 2018 at 23:37 Hours UTC # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23614?src=pr&el=h1) Report
> Merging [#23614](https://codecov.io/gh/pandas-dev/pandas/pull/23614?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/8af76370a10399b539e93e2533ad7f25d77263c5?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/23614/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/23614?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #23614      +/-   ##
==========================================
- Coverage   92.25%   92.25%   -0.01%     
==========================================
  Files         161      161              
  Lines       51384    51383       -1     
==========================================
- Hits        47405    47404       -1     
  Misses       3979     3979
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.64% <100%> (-0.01%)` | :arrow_down: |
| #single | `42.31% <100%> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/23614?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/formats/format.py](https://codecov.io/gh/pandas-dev/pandas/pull/23614/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2Zvcm1hdHMvZm9ybWF0LnB5) | `97.87% <ø> (-0.01%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/23614/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.02% <100%> (ø)` | :arrow_up: |

------

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> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23614?src=pr&el=footer). Last update [8af7637...ddf42c7](https://codecov.io/gh/pandas-dev/pandas/pull/23614?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Looks quite nice.

I think it good be good to have one test to see that `decimal` is working in `to_string`, and then we need to add to the whatsnew two entries, one for the addition of decimal, and another one for the argument resorting. @datapythonista Please let me know the two entries are not in the right place in `whatnews` file. @jreback  All checks are passed. Could you re-review this ? Thanks. lgtm. @datapythonista merge when you are satisfied. thanks