Allow for both nozzles to be purged every layer

While this is not an absolute fix, as more than 2 nozzles would not be supported, it would purge the two first nozzles every layer.

A better fix would to send nozzle[i] to skirt immediately before printing out nozzle[i]'s support/perimeter/infill/etc. This would purge the nozzle immediately before you print to the main model.

ie:
for layer i:
  support nozzle[1] -> skirt nozzle[1]
  print skirt nozzle[1]
  print support nozzle[1]
  perimeter nozzle [0] -> skirt nozzle[0]
  print skirt nozzle[0]
  print perimeter nozzle[0]
  print infill nozzle[0]
  i++

Not sure how to do this, but it would be a great addition.

Thank you,
TJ Mustard