Adds client-side plugin to onebox youtube videos.

Supports:
- www.youtube.com/watch?v=(id)
- www.youtube.com/watch?foo&v=(id)
- www.youtube.com/watch?v=(id)&foo
- www.youtube.com/watch?v=(id)#t=foo
Adds the width handling. Default of 640, shrink if necessary.

The default is what youtube uses. The shrinking is if the screen
is too small. (mobile, window sticked to the side, etc.)
Merge branch 'youtube' of https://github.com/Ralt/miaou into Ralt-youtube
major style change
fixed a few problems with the youtube renderer

It was working on m.content, thus cancelling the work of previous renderers (including the markdown renderer). This also made it unable to find the <br>, which is why it couldn't work on multiline message. As it was returning true, it was also preventing other post-renderings.
Merge branch 'Ralt-youtube'
Merge branch 'master' of github.com:Canop/miaou