Ability to request extra fields when using collection utility

Previously the initial load would retrieve CollectionAlbum objects and
when processing "add to collection" events, it would retrieve
Album objects. This causes an issue if you're requesting
CollectionAlbum-specific fields (i.e. itemTrackKeys).

Now calls to the `get` API method use the CollectionAlbum key instead.
This is done by adding a `collectionKey` to the stored album object.

cc @iangilman 
What does the format look like if you want multiple extras? Should show an example here.