Add highlighting support for code blocks

This code is somewhat messy because it has to work around the extreme
limitations of the xmldom window polyfill provided by Lemon.

- Updates the dependency on Beorn to 0.2.1 and adds a new one for
  highlight.js at 11.8.0.

- Defines a wrapper build script to set up a transformation hook and
  then dynamically run Beorn through an import.

- Updates .rsync-filter to exclude these new files.