Add support for custom frames

[randoragon]

All ID3v2 standards (ID3v2.2, ID3v2.3, ID3v2.4) state that frames beginning with identifiers X, Y and Z can be freely used for user-defined purposes. It would be cool if rsid3 had an interface that would allow the inspection, creation, deletion and editing of such frames.

This could be a pretty big undertaking and I doubt it's a feature I will personally need, but it is something to consider.