Feature request: option to output `$` or `\(` instead of `<x-equation>` in html output

We use `md4c` to process markdown with LaTeX formulas which is fed to MathJax, which prefers `$` or `\(` directly. I have a working implementation <https://github.com/acmepjz/md4lean> which added two options `MD_HTML_FLAG_MATHJAX` and `MD_HTML_FLAG_MATHJAX_USE_DOLLAR` to HTML output. Do you think it's a good feature to be added? If yes, I can create a PR for it.

[EDIT] PR created as #284.