ENH: added an optional css id to <table> tags created by to_html()
This is useful for the reasons mentioned in [#8496](https://github.com/pandas-dev/pandas/issues/8496) 
- [x] closes #8496
- [x] tests added / passed: `test_to_html_with_id`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry
