Support for Multistream Encoder/Decoder API

[tortis]

libopus offers a Multistream API which allows for more channels to be packaged into an opus stream. How difficult would it be to expose those? And is it within the scope of this project?

Thank you!

[tortis]

I implemented the multistream decoder with a copy-paste job to see if it would work, and it does! for my use-case at least.

https://github.com/musesessions/opus
https://github.com/hraban/opus/compare/v2...musesessions:opus:v2?expand=1

I'm open to contributing a more complete implementation, but I'd like feedback on how it should be done. Is the copy-paste acceptable, or would you prefer the golang Decoder struct to internally handle regular/multistream? If the copy-paste is good, then do you want it in a separate file?