source highlighting (CodeRay & highlight.js)

- source highlighting based on source-highlighter attribute for HTML backend only
- if safe mode is SECURE or greater, document *cannot* activate source highlighting
- CodeRay for direct server-side highlighting
- highlight.js for client-side highlighting
- both highlighters require some stylesheet tweaks to work w/ default AsciiDoc styles
Merge pull request #52 from mojavelinux/source-highlighting

Source highlighting (two forms)