Add highlighting support for code blocks

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.