Use more than one core

[Profpatsch]

At the moment the information extraction only uses one core max; a -j option would be nice since it could speed the whole thing up by orders of magnitude.

See https://github.com/sampsyo/beets/blob/master/beetsplug/convert.py for an example how to implement something like this.

[JonnyJD]

Yes, using multiple cores without using additional tools would be nice.

This should be made optional (or have the possibility to only use one core, -j is fine).
I tend to use only 1-2 cores since it even runs for a long time when I do use all 4 cores, but with 1-2 cores my computer is usable over that time. With 4 cores it is not.