Firefox compatibility

[raphaelbastide]

I managed to make the script Firefox compatible, by replacing those lines by:

  navigator.mediaDevices.getUserMedia(constraints)
  .then((stream) => {
      gotStream(stream)
  })
  .catch((err) => {
      alert('Error getting audio');
      console.log(err);
  });

I can’t really PR because I can’t get adapt gotSources() properly with MediaDevices.enumerateDevices()