adding writers, primary_artists and producer_artists ton Song

[VerbalKint95]

First time participating in a project so I don't know how to do pull request...
Just want to add those lines in Song constructor to gather more information.

self.primary_artists = [Artist(client, artist_data) for artist_data in body.get("primary_artists", [])]
self.writers = [Artist(client, writer_data) for writer_data in body.get("writer_artists", [])]
self.producers = [Artist(client, producer_data) for producer_data in body.get("producer_artists", [])]
self.featuring = [Artist(client, featuring_data) for featuring_data in body.get("featured_artists", [])]

[johnwmillr]

Thanks for the suggestion! This has been implemented in #334.