Added support for embedded radd.it playlists.

Self-contained in its own function, added support to embed in-line radd.it links.
I assume you mean line 3934?  It's just stripping out everything before (and including) the "radd.it" in the URL to get the permalink.  (So radd.it/r/blah becomes /r/blah, etc.)  Possible domains are radd.it, m.radd.it, and embed.radd.it.