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 

reindexing tests like these added? thanks @attack68 
can you do a followon which updates the notebook to use this feature (or is this pretty esoteric)? @jreback I have a few Styler PRs being worked, plan is to update the docs notebook after they all get finalised, preferably well before 1.2 release? I can link them with each other, since there will be multiple ways now of achieving same results.