Syntax change: Use video ID instead of URL

I have proposal: lets use video ID instead the full URL:

```.. youtube:: f00BarSPaM```

This makes the code cleaner and allows to eliminate the ```get_video_id``` function from the code.

Also, it makes your syntax more compatible with the one used in the youtube extension in the sphinx-contrib repo: https://bitbucket.org/birkenfeld/sphinx-contrib/src/940757f22e335f6ae98521621201683d6bab1d60/youtube/?at=default