Immediately start on previous

[joshbetz]

The previous button functionality is not optimal

Currently:

1. build playlist
2. initialize player with playlist
3. delete songs from front of queue until we get to currentIndex
4. play

How it should be:

1. Initialize player with previous song
2. play
3. build playlist
4. delete songs off front of playlist until currentIndex + 1 is the next song
5. add items in playlist to queue

[joshbetz]

This would let the player start playing right away and essentially enqueue the rest of the songs in the background instead of waiting to rebuild the playlist before we can start playing music.