Add music support

[puzzud]

Add the ability to play music with each voice on each platform:

• 3 C64 voices
• 4 NES voices

The PCM voice on the NES does not need to be implemented just yet.

There should be a zero page variable that is:

• 0 when music is not playing or should be stopped.

• 0 when music is playing. It should also indicates how many times the music should play.

• -1, 0xff when music should play in a loop indefinitely.

[puzzud]

My project puzlar has an unpublished music engine that could be used.