Add support for exporting VT file format for playing music on real Vic20

[furroy]

There already exists a tracker VicTracker and a tool vtcomp for making music on real Vic20 hardware. VicTracker is designed to run on real hardware, so it's quite limited in features. It would be great if Furnace could be used to compose the music instead. VT/vtcomp use a fairly simple file format for storing music, so if Furnace could export this vt format, the existing VT tools can be used to either make a .prg file or the asm for an irq player so the songs can be incorporated into your own demos/games.

vtcomp usage

vt file format