Add new LaTeX Extra

[navanchauhan]

This extra converts inline and block latex in markdown to MathML

Closes #366 and #134

GitHub Gist with example

[navanchauhan]

Oh I think I messed up the setup.py trying to add the extra dependencies

Edit: Manually tested by running python -m pip install git+https://github.com/navanchauhan/python-markdown2@latex-extra

[nicholasserra]

Thank you! Could you add a quick test for this new extra format?

[navanchauhan]

Updated the extra to disable rendering between anything wrapped with

,  `, and ```

Also added tests for the same

[nicholasserra]

LGTM thanks!