Warn about a channel specifier on video.

[na-g]

@na-g do you think enconding parameters are in general allowed in video rtpmap's by the spec?

I guess I would prefer to leave this in as a warning if possible (instead of removing the check all together), but I don't know if that is going to cause more problems.

Originally posted by @nils-ohlmeier in https://github.com/mozilla/webrtc-sdp/pull/196#issuecomment-562701576

[na-g]

I do wonder if multichannel video will be useful for VR eventually.