Add AMS macros to the mathjax pipeline.

There are other mathjax components that would be useful to some users as well, and I can't assess whether or not this is a generally useful change, but it's essential to our use of mathjax with Antora (we use KaTeX via a custom Ruby extension in our usual asciidoctor builds).

Ideally additional packages such as ams could be specified via a global or block-local attribute set on latexmath blocks, but that's a bigger project.

Fixes #14 (closed)