ENH: add set_td_classes method for CSS class addition to data cells
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Simple extension to Styler to allow cells to be assigned CSS classes from a DataFrame of strings.
Seems to be a reasonably common request.
The existing framework, i.e. the `cell_context` dictionary (which previously did nothing) is leveraged.

@TomAugspurger 
