Add support for sending config options to VMM.MediaElement.create().

This is just a quick proof of concept for a feature I needed in a project.

I'm using the VMM library outside of a TimelineJS context, and need to be able to specify the size of YouTube elements. The size is currently hardcoded into `VMM.ExternalAPI.youtube.create`. This PR allows a third parameter to `VMM.MediaElement.create`, called `config`, that is passed on to the media element.

I imagine this could be useful for other content types as well, but I want to get a quick feedback on whether something like this is likely to be accepted before I spend more time investigating it.

As it stands, this should be 100% backwards compatible, but will of course "reserve" the third parameter of `VMM.MediaElement.create`, and thus limit future flexibility.