Backs up all public spotify playlists for a given user to text files contains Spotify track URIs which can be copied and pasted back in to Spotify to restore a playlist
Requires \$SPOTIFY_USER be set in the environment or else given as the second arg
Requires \$SPOTIFY_ACCESS_TOKEN in the environment (can generate from spotify_api_token.sh) or will auto generate from \$SPOTIFY_CLIENT_ID and \$SPOTIFY_CLIENT_SECRET if found in the environment