PR for API to do simulcast with RID according to Plan X

I have written a PR reflecting "Plan X", which allows the app to specify
encodings in addTransceiver:

https://github.com/w3c/webrtc-pc/pull/353


Obviously, this isn't of full value until addTransceiver PR has been
merged, but I believe this PR is complete enough to review.  There is an
example of how it can be used in the PR, where this is the relevant part:

            pc.addTransceiver(track, {
                send: true,
                receive: false,
                sendEncodings: [
                    {
                      rid: "f",
                    },
                    {
                      rid: "h",
                      scaleDownResolutionBy: 2.0
                    },
                    {
                      rid: "q",
                      scaleDownResolutionBy: 4.0
                    }
                ]);


I choose the names "sendEncodings" and "scaleDownResolutionBy", but if
anyone has a better idea of what to names those, suggestions are welcome. I
erred on the side of verbose.