Switch to Highlight.js

As discussed in #134
Add Support for Choosing Pygments or Highlight.js

Just add `"highlighter": "pygments"` to your `.groc.json` or use the
`--highlighter=pygments` flag.
Merge branch 'patch-highlightjs' of https://github.com/killercup/groc into killercup-patch-highlightjs

Conflicts:
	package.json