Add a Youtube widget

There are still some commits to come to get it to work with our new development environment,
namely that the plugin is being loaded from outside of the ckeditor plugins folder, such as updating `test.html`.
I'd turn this into a private method for example

buildYoutubeUrl (embed, settings) ->
    src = getParam(embed, 'src') || settings.src
    type = getParam(em....
    url = 'http://youtubaelsjfd'
    return url + '/' + ...