add audio to the game

[christopherbot]

Add audio to the game.

• Add a music track
• Allow player to mute the music
• Record multiple versions of the track (e.g. one chiptune, one instrumental) and allow player to toggle audio between the two while keeping the current position in the song. The songs would have to line up exactly.
• crossfade the audio when switching tracks
• Add sound effects to pick-up-food and hit-ball