Implement new playlist endpoints

[andrzejressel]

Few months ago Spotify added changes to playlists: https://developer.spotify.com/community/news/2018/06/12/changes-to-playlist-uris/.

To implement

• [ ] GET /v1/playlists/{playlist_id}/tracks

• [ ] POST /v1/playlists/{playlist_id}/tracks

• [ ] PATCH /v1/playlists/{playlist_id}/tracks

• [ ] PUT /v1/playlists/{playlist_id}/tracks

• [ ] DELETE /v1/playlists/{playlist_id}/tracks

• [x] ~~GET /v1/playlists/{playlist_id} (PR)~~

• [ ] PUT /v1/playlists/{playlist_id}

• [ ] POST /v1/me/playlists

• [ ] GET /v1/playlists/{playlist_id}/images

• [ ] PUT /v1/playlists/{playlist_id}/images