Finish PlugMedia

[ylt]

Implement youtube and soundcloud querying in PlugMedia (within plug plugin). This will allow more plugins to use the functionality. This will also allow additional functionality such as validating the data (such as length) before adding the playlist.

Another advantage of implementing it at this point is for if Plug decides to implement other music sources. It can just be added without having to change any other plugins (especially if I make a consistent interface within PlugMedia itself for retrieving this info)