Suggestion to add a -a album option

[j-peeters]

I really like this!
A suggestion for an addition to the functionality of this plugin would be to add a album -a option so that one can check the stats for album genres with
beet describe -a genre

Thanks for considering!

[adamjakab]

Nice idea @j-peeters.
I am a bit short on helping hands and this probably requires a full code review as this was never considered.
I will keep this open until we get it done.