Avoid being specific to mathjax.
Add KaTeX to the distribution.
Switch to KaTeX for math.

Unlike MathJaX, KaTeX is small enough to be included directly in the
distribution; the JS is 110KB, the CSS is 20KB, and the full suite of
fonts are about 850KB.
Use LaTeX in a few of the standard lib crates.
Allow LaTeX in standalone markdown files via --markdown-enable-math.

Requires the KaTeX files to be arranged correctly.