ENH: `Styler.highlight_null` can accepts `subset` argument
As proposed in #31345 and as a follow-up PR for [#29118](https://github.com/pandas-dev/pandas/pull/29118#discussion_r340733887)

- [x] closes #31345
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry
