Remove or make optional the 2-channel limit

2-channel limit we've imposed arbitrarily breaks some multichannel files. Should either make this optional, remove it and just send all data out, or change to down mixing.

https://github.com/brion/ogv.js/issues/293
https://github.com/brion/ogv.js/issues/241
https://github.com/brion/ogv.js/issues/30
