Changed youtube regex to match id-s in more links

This regex is a little slower but matches every possible youtube link format I could find.

Found in [this answer](http://stackoverflow.com/questions/8388223/youtube-url-id-regex).

[Tested here](http://www.phpliveregex.com/p/30A).

Since I have no development tools here (github edit button is all I need), I haven't tested that my commit works, but there weren't many things to mess up so I'm optimistic.

The only thing it doesn't match but should is `[youtube]1nV2EocffQ0[/youtube]`.