Use MathML for mathematics on posts

We're finally in an era where this is natively supported on all major
browers and platforms, at least on modern browsers.  I've been waiting
for this for...nearly 15 years now?  My go-to previously was Mathjax.

I guess I should remove my IE11 support in CSS, then, huh?

Or maybe leave it around as an easter egg / relic type of thing.

src/post2html

@@ -180,6 +180,7 @@ main()
          --metadata pagetitle:ignoreme \
          --wrap none \
          --highlight-style tango \
+         --mathml \
          -B <( src/mkheader post @__PAGE_TITLE__@ ) \
          -A src/footer.tpl.htm \
          < <( prefmt "$file" ) \