MathJax support in texi2any

[Gavin Smith]

I've implemented basic MathJax support in texi2any in commit
7ff3cbdc4. To use it, run texi2any as "texi2any --html -c
HTML_MATH=mathjax". Feedback would be appreciated on how well it works
with Texinfo manuals containing math typesetting.

I think the main concern at the moment is making sure that the source
code for MathJax is easily accessible from pages that reference it,
and that users do not need to be dependent on MathJax's hosting.