Join saved tracks / audio features with Track response object

[sspenst]

I've written an app with the Spotify API and with each track I display its audio features and whether or not it is a saved track. It would be nice if this data could be optionally combined with the Track response object. I imagine the option to join /me/tracks/contains booleans with tracks would be especially useful, as displaying if track has been liked is a common use case.

This would benefit both Spotify and 3rd party apps:

- 3rd party apps only have to make one request for relevant data

- Net time to get all relevant data should be faster when combined into one request versus 2 or 3 separate requests

- Spotify receives fewer requests from users and makes fewer database round trips, meaning reduced server load / faster response times