Barplot add space option
- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] Adds `space` option to bar charts, which adds space between columns. This is nice especially if the user is making a grouped bar chart.

Example figure
![example](https://user-images.githubusercontent.com/7705285/47348034-3ddd3080-d6b9-11e8-8380-3ada4f8b6cbe.png)
:

Hello @Pingviinituutti! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/plotting/_core.py`](https://github.com/Pingviinituutti/pandas/blob/e909f0d76c2fe4fed73fcfb4ae7421c14d6fa9d0/pandas/plotting/_core.py) !

 Good comments! I'll allocate some time to add that stuff :) # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23292?src=pr&el=h1) Report
> Merging [#23292](https://codecov.io/gh/pandas-dev/pandas/pull/23292?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/14f25bb8b7ba0c709dd36d79966abea648d0f48b?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/23292/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/23292?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #23292      +/-   ##
==========================================
+ Coverage   92.22%   92.22%   +<.01%     
==========================================
  Files         169      169              
  Lines       50900    50902       +2     
==========================================
+ Hits        46943    46945       +2     
  Misses       3957     3957
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.65% <100%> (ø)` | :arrow_up: |
| #single | `42.29% <0%> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/23292?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/plotting/\_core.py](https://codecov.io/gh/pandas-dev/pandas/pull/23292/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb3JlLnB5) | `83.65% <100%> (+0.02%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23292?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/23292?src=pr&el=footer). Last update [14f25bb...944e565](https://codecov.io/gh/pandas-dev/pandas/pull/23292?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @Pingviinituutti can you merge master, update to comments, and add a test Closing as stale. Ping if you'd like to continue