x/crypto/ssh: No support for Window Dimension Change Message

Please answer these questions before submitting your issue. Thanks!

1. What version of Go are you using (go version)? go version go1.6.1 darwin/amd64
2. What operating system and processor architecture are you using (go env)? darwin/amd64
3. What did you do? If possible, provide a recipe for reproducing the error. A complete runnable program is good. A link on play.golang.org is best.

Simply a part of the RFC 4254 document that was not implemented; would want a method on an ssh.Session object to send messages such as this.

1. What did you expect to see? N/A
2. What did you see instead? Not yet implemented.