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

[acmepjz]

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.