ENH: should users be able to adjust subplot parameters?
Noticed this while reviewing #34343

Currently, the example in the doc would look like this:
![image](https://user-images.githubusercontent.com/33491632/82753854-0799f280-9dc1-11ea-9675-7716dd75162f.png)

IMO there's insufficient separation between the subplots. If we change [this line](https://github.com/pandas-dev/pandas/blob/cb35d8a938c9222d903482d2f66c62fece5a7aae/pandas/plotting/_matplotlib/boxplot.py#L427) to
```python
        fig.subplots_adjust(bottom=0.15, top=0.9, left=0.1, right=0.9, wspace=0.2, hspace=.5)
```
then this example would look like this
![image](https://user-images.githubusercontent.com/33491632/82754046-44b2b480-9dc2-11ea-97ad-a13b58d0d218.png)


It seems these subplot adjustments aren't adjustable by the user.

So, should the user be allowed to pass in these parameters (e.g. `hspace`), or should the fix here be to just hardcode `hspace=.5` into the line mentioned above, and if users want extra customisability they can use matplotlib directly?