Add support for metron.cloud as alternate metadata provider

[CornyMovies]

Add metron.cloud as an alternative metadata provider.
metron.cloud supports searching using cv ids, so I think the "only" things needed is to write a class like the ComicVine one, add a check box to switch between ComicVine and metron.cloud, and maybe add the metron_id to the database.
The reason to do this is that the ComicVine API has been down for a couple of days now with zero communication about when it might be available again.

[CornyMovies]

It wasn't clear but I'm volunteering to this

[Casvt]

Let's give CV some time to fix their API first. If it's fixed soon, we'll implement this later as an extra feature. If the API is not coming back (which I doubt), then we'll hurry a bit more to get this working.