Support tex_math_dollars extension

[Omikhleia]

The title says it all, with Pandoc-like tex_math_dollars, one may have $ math formula $ or $$ math formula $$.

EDIT: I mean here, to clarify, minimally, the reader (parsing) support for it, and the basic generic writer handling methods. Transforming the input in some appropriate format in the writers (e.g. MathML in HTML ?) could be left to other implementers.

[Omikhleia]

For the record, I initiated the lunamark reader side of things something here https://github.com/Omikhleia/markdown.sile/pull/23/files

(EDIT: Quick'n'dity attempt obviously, but I thought it might be worth mentioning, while I'm struggling on the SILE side of things - so people can check, and we can perhaps move forward with a PR independently).

[Omikhleia]

Update: Did progress a bit on the above-mentioned PR on my "vendored" copy - but struggling with differences of interpretation between GitHub and Pandoc, and idiosyncrasies. I added my comments there, until I get the time to pursue the effort.