Add `noHighlightCSS` conf option to omit the highlighting css

This is useful if a custom stylesheet already has the highlighting definitions integrated.
Split out google-code-prettify
Update to latest version of google-code-prettify
Use google-code-prettify as AMD module

The latest versions are now AMD compatible, as long as the module name
`google-code-prettify` is used. Updated Highlight.js to use the
exported object.
Merge pull request #288 from KevinCathcart/pretty-print

Pretty print