code: github.com/a1k0n/jsxm
tune: my dirty old kamel - Zalza
todo:
 - unroll short sample loops
 - unroll mixer code loops
 - pingpong loops
 - multi-sample instruments
 - instrument fadeout
 - missing XM effects (in rough order of priority):
   - Rxy - retrigger
   - Bxx / Dxx - position / pattern jumps
   - 7xy - tremolo
   - E3x, E4x, E5x, E6x, E7x, E9x, EDx, EEx
   - Gxx, Hxy, Kxx, Lxx, Pxy, Txy
 - oscilloscopes