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.

[danielfrg]

This lgtm.

could we do a small change to not create a new Exporter and instead do it with a variable, something like:

default_mathjax_url = "https://cdnjs.cloudflare.com/ajax/libs/mathjax/2.7.7/latest.js?config=TeX-AMS_CHTML-full,Safe"
mathjax_url = custom or default_mathjax_url

[danielfrg]

Maybe even make the default of custom_mathjax_url to be "https://cdnjs.cloudflare.com/ajax/libs/mathjax/2.7.7/latest.js?config=TeX-AMS_CHTML-full,Safe"