add possibility to provide the mbid of the release as a parameter

Other tools (rippers?) might want to select the correct release in other ways and could then use isrcsubmit for submission semi-automatically.

morituri possibly wants to do it this way.

This is a step between using isrcsubmit "bare" and using python-discid/pymbngs directly.
isrcsubmit still has more backend options and some duplicate/error detection.

Anyways, having this possibility doesn't really hurt.