Switch to Highlight.js for syntax highlighting

This replaces Rainbow with Highlight.js for syntax highlighting.
Highlight.js supports more programming lanuages and has more styles
than Rainbow.

This also improves how vendor scripts are included into the
checked-in source tree.
Remove Highlight.js styles
Merge pull request #152 from ericf/syntax-highlighting

Switch to Highlight.js for syntax highlighting
Upgrade to broccoli-es6-module-transpiler@0.5.0

This improve sourcemaps on the site by upgrading to the latest version
of the transpiler.