Feature: clickable folders in table of contents

Well, what else should I write ? … it was too annoying, so I implemented clickable folders in the table of contents. Everything works as expected - including keyboard-navigation. (I didn't touch it anyway). I hope I didn't miss anything … well I did not test empty folders as I was not able to reproduce such a combination (empty but visible).

This should also fix: https://github.com/nevir/groc/pull/80 …

So I stay calm and look forward to a successful merge of this pull-request.

Stephan :zzz: