support a latex backend for latexmath instead of mathematical

[cod3licious]

I'm using asciidoctor-pdf with asciidoctor-mathematical and the equations are really ugly compared to standard latex. It seems like it shouldn't be too hard to convert latexmath equations into prettier images using latex directly.

Could this maybe be added as an additional backend or something, especially since mathematical also seems quite buggy (not rendering underbraces correctly, etc)?

[infinity0]

See #117. I've tested it locally and it works well.

Note that latexmath doesn't support inline output at the moment, but it will do once they merge my PR plurimath/latexmath#44.

OTOH, there are also quite a lot of other bugs like plurimath/latexmath#43 and the project is not very active.

Also, mathematical itself is no longer maintained.

The latex2mathml python module in docutils generally works much better and with less bugs, so perhaps calling that from Ruby is a better approach than either ruby-mathematical or ruby-latexmath, from the perspective of the end user.

A version of Chrome in the near future will have MathML support behind an experimental flag so the future is looking bright for MathML in general, so it's worth pursuing to have the functionality in asciidoctor.

[infinity0]

Apparently plurimath is the successor of latexmath. However, it has some regressions from latexmath, so that is not reassuring either.

I added a commit to #117 to instead use python-docutil's latex2mathml, as it is much less buggy, and already supports inline math.