MathJax 3 support

[voidbert]

Addressed issue #1602.

A new option, --mathjax-version=VERSION was added. This allows the user to specify what major MathJax version they want to use. This information can be used to choose the correct file (from the Cloudflare defaults), and to initialize the library correctly (2 and 3 have different configuration methods), even if the user provides their own files.

This is my first PR in this project, so please tell me if my code may break anything and how it can be improved. The HTML tests don't pass, but I tested that that was already the case in the commit before mine.