Permit setting 'style' to default, and add 'bbox' as 'border' alternative for inner a-b-c labels and titles

[pratiman-91]

This is my first contribution. Let me know if there is any problem with the issue or failing any tests.

[lukelbd]

Hi @pratiman-91, thanks for the PR! That bugfix is easy and I like the idea of "bounding boxes" for titles and a-b-c labels. When you get the chance can you fix the PEP8 errors? Also you might consider installing pre-commit, which will show you the PEP8 issues and prevent you from commiting until they are fixed, as mentioned in the contribution guide.

[lukelbd]

(Fixes #234)

[lukelbd]

Apparently I'm allowed to edit your PR myself (never did that before!). This was close to being ready anyway so I decided to fix the issues. Thanks again, merging now

[lukelbd]

I made some improvements to this feature on master (4288a73):

1. "Borders" are still used by default (because IMO borders are less obtrusive and I see big no reason to change the default).
2. By default a square bounding box is drawn perfectly flush against the subplot edge.
3. There are new title.bboxpad and abc.bboxpad options to control padding.
4. Turning on title.bbox turns off title.border, and vice versa. You cannot have both of them on at the same time... which I imagine is what users will want 99% of the time.

Here's a nice example with these new features:

import proplot as plot
fig, ax = plot.subplots(ncols=2)
print(plot.rc.abcborder)  # prints True
ax.contourf(np.random.rand(10, 10))
ax.format(abc=True, abcloc='ul', abcstyle='A.', abcbbox=True)  # temporarily sets border to False

[pratiman-91]

Looks good.
Thanks!