Switch to Marked.js

[Marked.js](https://github.com/chjj/marked) is a bit faster than Showdown (9s vs 9.5s rendering groc's documentation) and offers easy code highlighting (using a callback to highlight.js).

This changes the `style.css`, which is only compatible if you applied #149 first.

I also removed the 'showdown' CLI option, but this could probably be replaced by marked option if wanted.

See #134 for a related discussion as well.