Add prefix to BioGRID config

Tell @JoelRichardson

I added a config option - "prefix". This appends the specified value to the identifier. Also, maybe we could ask BioGRID to change their identifiers to have "MGI:".

I also made the ID resolver valid for all organisms, not just human. I don't remember why that was there but it's a bit hacky.

Refs #809