Allow various codecs.

It is now possible to specify codecs other than VP8 and Opus.  This
turns out not to be very useful, since VP8 is the only codec supported
by all browsers (in violation of the WebRTC spec, which mandates support
for H.264), and there is no good reason to use anything other than Opus
for audio.

conn/conn.go

@@ -17,6 +17,7 @@ type Up interface {
 	DelLocal(Down) bool
 	Id() string
 	Label() string
+	Codecs() []webrtc.RTPCodecCapability
 }
 
 // Type UpTrack represents a track in the client to server direction.