HTTP/2 push support (golang 1.8) (#1215)

* WIP

* HTTP2/Push for golang 1.8

* Push plugin completed for review

* Correct build tag

* Move push plugin position

* Add build tags to tests

* Gofmt that code

* Add header/method validations

* Load push plugin

* Fixes for wrapping writers

* Push after delivering file

* Fixes, review changes

* Remove build tags, support new syntax

* Fix spelling

* gofmt -s -w .

* Gogland time

* Add interface guards

* gofmt

* After review fixes

caddyhttp/httpserver/plugin.go

@@ -459,6 +459,7 @@ var directives = []string{
 	"proxy",
 	"fastcgi",
 	"cgi", // github.com/jung-kurt/caddy-cgi
+	"push",
 	"websocket",
 	"filemanager", // github.com/hacdias/caddy-filemanager
 	"markdown",