Embed item data into web request for `/item/`

The mopidy beets client initially [fetches all tracks at once](https://github.com/mopidy/mopidy-beets/blob/master/mopidy_beets/client.py#L67). This means one HTTP request per track. With this commit one can request `/item/?embedded` and receives as JSON document with all tracks under the `items` key. This is similar to the `/item/query/` request.