added ID3v2.grouping ("content group description", "TIT1"/"TT1" frame)

Support grouping (`TIT1`). Here's the description from the spec:

>TIT1
>   The 'Content group description' frame is used if the sound belongs to
>   a larger category of sounds/music. For example, classical music is
>   often sorted in different musical sections (e.g. "Piano Concerto",
>   "Weather - Hurricane").