Add barcode metadata field

[srussel]

Musicbrainz includes the CD barcode with the album metadata. It would be great if beets could store this barcode when available as I would like to be able to lookup albums by scanning the barcode of a CD.

[jackwilsdon]

It seems like there is a third-party plugin that may help you here, named beets-barcode, although I don't see why we couldn't store the barcode too.

[sampsyo]

Added to MediaFile in beetbox/mediafile#27.