Create math-katex filter

[benabel]

Render math with katex server-side.

[tarleb]

Thank you for this! Looks like a very useful filter, and the code is beautifully clean, too.

We're currently discussing the future path of this repo, and I'd like to wait for the discussion to reach a conclusion before merging. Please feel free to add your voice there, too: #207

[timtroendle]

@benabel, this looks very useful to me. Is your filter available somewhere?

[benabel]

@timtroendle in fact I never published it. You can find it on my fork https://github.com/benabel/lua-filters/tree/master/math-katex.