Include genre hierarchy

[MxMarx]

It could be useful to retain the genre hierarchy returned by Audible (#432 ) in addition to splitting categories into top-level (genres) and non-top-level (tags). This would make it easier to organize media is a more granular way by using broader and more specific tags.

To maintain compatibility, maybe this could be done by adding a field for a tag's parent genre/tag ASIN in the genre field.

If this is acceptable, I would be happy to implement it!