Add "Private Session" support

The original Spotify client offers to enable a private session so songs aren't tracked in the account statistics. It would be awesome if Psst could support this option. Preferably with persistence between restarts cause that's something the original client doesn't support which is kind of annoying.

Thanks for this great project! I'm looking forward to a fully-featured, native and not-so-memory-hungry Spotify client :)

Tracks are currently not reported at all.