Add podcast support

[felix-hilden]

Podcast support has been announced by Spotify! Let's implement it right away.

It has some quirks though, which I've addressed on an issue in spotify/web-api#1511. So let's wait a bit and see if they get addressed or changed. Also, some player and playlist endpoints provide a compatibility parameter additional_types. We could be better off without it, but I'm not quite sure yet.

• Add proper documentation
• Changelog
• New podcast endpoints
• Podcasts in player
• Podcasts in playlists

It seems that playlist endpoints have some issues. The additional_types argument defaults to returning episodes as track-like objects. We could:

• release our podcast support simply with the new endpoints and player changes
• get rid of the argument entirely and enforce additional_types=['track','episode']
• default to tracks and episodes, but parse to dicts if the argument is specified, much like fields

Then there are additional things which might be problematic as well.

• show episodes can be None when getting multiple shows
• episodes can be None in episode search

Side note, trying to implement the playlist functionality brought up a problem in our handling of the fields argument in playlist and playlist_tracks. It will be resolved here as well.

[felix-hilden]

The additional_types argument has this behavior:

• None: tracks returned, episodes returned as tracks
• ['track']: as above
• ['episode']: tracks returned, episodes returned
• ['track', 'episode']: tracks returned, episodes returned

So track has no effect, as it is the default accepted type. episode enables episode objects to be returned. We could modify things a bit. In effect it could be replaced by episodes_as_tracks: bool or something similar. The documentation mentions the possibility of new types in the future. But I can't imagine what they could be, so that probably is far in the future. If new types are added, we can then react by changing the argument. I think this is a better experience for users of this library as it makes the effect very clear.

Similarly for playback endpoints the parameter could be tracks_only: bool, as it returns None values if episode is not given.