Made the YouTubeIE regex verbose/commented

[FiloSottile]

This might be useful both to regex beginners (our regex is incredibly complete of language features!) and to new contributors.
Pretty sure i haven't broke it ;)

See http://www.regular-expressions.info/freespacing.html and https://gist.github.com/2176911

[FiloSottile]

@phihag I know I am pushing out a number of pull requests that will be a nightmare to merge, so simply give me the OK on the ones you think and I will do the merging :)

[phihag]

This one needs to be updated, and you're welcome to treat the current regexp to this. I'm not sure why def suitable is added here though.

[FiloSottile]

Ok, great, will do ASAP.

A change to that function was needed to add the re.VERBOSE flag; there were two ways to do that, changing the main suitable func, but that could lead to strange surprises with other regexs containing #'s, or by hacking it in YouTubeIE. Hope it is not a too ugly fix.