Some plugins such as "Additional Artists Variables" register variables with the information coming from MusicBrainz, however the current plugin registration locks the `TagVar.is_from_mb` flag as `False` so that the note on the help shows the message, "not provided from MusicBrainz data".  This is incorrect, and possibly misleading.

I propose to expose the `is_from_mb` flag as an optional argument to the `register_script_variable()` method, with a default value of `False`.