Transmit Stereo Audio

Since https://github.com/mumble-voip/mumble/pull/4209 has been merged (and will most likely be included in the Mumble 1.4.0 release) it would be nice to have the ability to transmit stereo audio implemented into gumble.

Presumably, at a quick uninformed glance, all that would need to change would be setting AudioChannels to 2 although it might be more complicated than that. https://github.com/layeh/gumble/blob/1ea1159c495624266a4adea43580748fd0fc9c57/gumble/audio.go#L30

Supposedly the "opus decoder can also automatically convert a stereo stream into a mono stream, so even if we transmit a stereo stream to an older version of mumble, it would still work." according to the author of the PR.

Regardless, it is probably sensible to default to mono still (since 1.4.0 hasn't released yet) with a configurable option to transmit in stereo.