Extend mbsubmit to run after initial import

[arsaboo]

Right now the mbsubmit plugin only works at the time of import. I am not sure if this is by design or just an oversight. It makes sense to be able to create a tracklist to be submitted even after the initial import.

Ideally, mbsubmit should be able to accept a query and print the tracklist to be submitted to MB.

[sampsyo]

Seems like an extremely reasonable extension (and not too challenging!) to me.

[arsaboo]

Indeed. I added another PR #4456 to your queue :)