Support TeX with KaTeX

[JoelOtter]

KaTeX is a great library for mathematical typesetting.

I had implemented support in commit 3b541317f2196bf51a648a03396ee7aac53ee04c, but had to remove it in fe67c6702208f25b3f7bb93542734bf6e12552a0 as there are issues that need to be resolved:

• Parsing the Markdown with embedded TeX is broken due to escaping in Markdown-It. https://github.com/waylonflinn/markdown-it-katex/issues/5
• There are some bugs with the parsing in general, though this PR looks promising: https://github.com/waylonflinn/markdown-it-katex/pull/4

[JoelOtter]

Looks like the PR is in - just waiting for it to be published to NPM and then I can add KaTeX.