%aunique fails when listing albums

[djl]

Problem

$ beet ls -a -f '$albumartist - $album%aunique{}'
Weezer - Make Believe<no such field 'album_id'>
Weezer - Maladroit<no such field 'album_id'>
Weezer - Pinkerton<no such field 'album_id'>
Weezer - Raditude<no such field 'album_id'>
Weezer - Weezer<no such field 'album_id'>
Weezer - Weezer<no such field 'album_id'>
Weezer - Weezer<no such field 'album_id'>

Looks like aunique assumes that it'll only ever handle Items (albums don't have an album_id field).

This seems like a simple fix. I should have a pull request ready today or tomorrow.

[sampsyo]

Sounds great! This seems like a nice, straightforward addition.