Add listener/orientation/velocity/doppler

This isn't a perfect PR, but I'd appreciate merge consideration as I think it adds quite a bit and I don't want to see our implementations diverge. I'm using this in my game and it works pretty well, adding true spatial audio where before only stereo was possible.

It may be a good idea to put deprecation warnings on pos/pos3d functions since I've eliminated them in favor of offset and position, the former to disambiguate completely and the latter to bring Howler more in line with the Web Audio spec.

I've also not updated the changelog because I'd like to hold off on doing that until everything is ironed out. The README should be up-to-date, though.