Support proxying HTTP/2 to backend applications

We're looking for a PaaS which supports proxying HTTP/2 to the backend app to enable mobile support for [gRPC](http://grpc.io). Deis supports terminating HTTP/2 at the router and opening an HTTP/1.1 connection to the app (#216) currently. The only L7 router that I know of which supports HTTP/2 to the backend is [nghttpx](https://nghttp2.org/documentation/nghttpx-howto.html) (with `--http2-bridge` in v1.8 or `backend=,;;proto=h2;tls` later). The other option is using a L4 load balancer to opaquely proxy the TCP stream. Felix says that the only tool he knows of to route to multiple TCP backends based on the SNI header is HAProxy.