Restructure Anchor handling + URL changes

I wanted to reference an episode via it's url, so I had to dig a
bit into the code… …and while I was there… …you know the drill. :-)

* Prefix URL hashes from `000` to `GST000`
* Change URL hash, when clicking episode links
* Handle invalid episode numbers
Merge pull request #1 from bascht/master

Got myself some linkable episodes.