Convert MathSpan to MathML using e.g. camoy/amath or gjtorikian/mtex2MML

For html (and odf and epub) it would be nice to be able to produce MathML as mentioned in fletcher/peg-multimarkdown#39

I.e. as an option, instead of wrapping TeX math for MathJax in https://github.com/fletcher/MultiMarkdown-5/blob/f2a0b9ce7446a1b45bd1bef2556dedcb0bfbecf5/src/html.c#L334-L352 convert this to MathML.

In the original issue [blahtex](https://github.com/gvanas/blahtexml) is mentioned, but doesn’t seem very active.

One very simple alternative might be to use [amath](https://github.com/camoy/amath), a peg-based AsciiMath to MathML converter. It’s only AsciiMath, so it might not be a good alternative for text that are based on TeX-like syntax, but the amath c/peg-code seem rather readable.

[mtex2MML](https://github.com/gjtorikian/mtex2MML), which is a active fork of [itex2MML](https://golem.ph.utexas.edu/~distler/blog/itex2MML.html), might be another alternative. It tries to be compatible with MathJax.