ENH: Add option to disable MathJax (#19824).
This prevents MathJax from processing table contents:

<img width="599" alt="screen shot 2018-02-24 at 7 36 50 am" src="https://user-images.githubusercontent.com/1804856/36632244-0c07bdac-1939-11e8-85eb-0c22d42991d1.png">

I'm hoping for a code review, to find out if this might be accepted. If so, I'll add the tests/docs.

- [x] closes #19824
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry