added custom_mathjax_url option

[dr-br]

Added the option to define a custom mathjax URL.
Not sure if all the modifications are really necessary in the code, please check! It also works with the privacy plugin of mkdocs-material.

Off topic: I'm not able to run the demo, some templates are missing in this repo. If I install mkdocs-jupyter via Pypi, the required template files are present.