Support handling CLI commands without a running application instance

[aome510]

Resolves #279

Changes

• initialize a new client and spawn a separate thread for handling CLI commands without a running spotify_player instance
• update CLI client code to support handling requests without the application's state
• move mute_state from PlayerState struct to SimplifiedPlayback struct

Benchmarks

Without a running instance

• playback next

$ time test_spotify_player playback next
________________________________________________________
Executed in    1.15 secs      fish           external
   usr time   28.03 millis    0.23 millis   27.80 millis
   sys time   18.36 millis    1.83 millis   16.54 millis

• get key playback

> time test_spotify_player get key playback
________________________________________________________
Executed in    1.27 secs      fish           external
   usr time   29.29 millis    0.25 millis   29.04 millis
   sys time   23.85 millis    2.13 millis   21.71 millis

With a running instance

• playback next

> time test_spotify_player playback next
________________________________________________________
Executed in   14.94 millis    fish           external
   usr time    7.58 millis    0.19 millis    7.39 millis
   sys time    5.54 millis    1.54 millis    4.00 millis

• get key playback

> time test_spotify_player get key playback
________________________________________________________
Executed in   16.28 millis    fish           external
   usr time    7.54 millis    0.19 millis    7.35 millis
   sys time    5.90 millis    1.63 millis    4.27 millis