expose PyGame sound looping feature

This is useful for psychoacoustics studies where you have a continuous masker sound that you want to play continuously throughout an entire block of trials; you can just create say 20 seconds of gaussian noise using numpy and create a sound with `loops=-1` (which means "loop forever until `stop()` is called).  Defaults to `loops=0` (PyGame's internal default) which means "play once and don't repeat."