Switch to Marked.js

It's faster than Showdown and offers easy code highlighting.

I'm not sure this works without my PR to switch to Highlight.js (because
the CSS changed quite a bit).
Merge branch 'patch-marked' of https://github.com/killercup/groc into killercup-patch-marked

Not safe

Conflicts:
	lib/cli.coffee
	lib/styles/default/style.css
	lib/styles/default/style.sass
	lib/utils.coffee
	package.json