ENH decimal parameter to to_latex and to_html, #12031
- [x] closes #12031
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

I just pass on the `decimal` parameter. All the machinery was already in place.

@jreback All set!
 thanks @nbonnotte 
