ENH: `styler.html.mathjax` option
This pandas option is added to allow the `Styler` options to be independent from the others. See #41395

 - [x] add option in config
 - [x] add tests
 - [x] whats new
 - [x] `options.rst`

will update styler user guide as a follow on when all options PRs merged.

![Screen Shot 2021-08-29 at 10 01 55](https://user-images.githubusercontent.com/24256554/131243266-a35e12be-43fa-4be8-8350-c538e32abd80.png)

can you rebase