API: Give an utility to MOODs

[Funasitien]

Audius has build in support for moods, which is a really interesting way to tag tracks, and so it would be cool if you could have them as parameters - in the search feature or in the auto-generated routes (like /v1/tracks/trending, /v1/tracks/trending/underground, /v1/playlists/trending, etc...)

[raymondjacobson]

Stay tuned! @sliptype @Kyle-Shanks