Add proxyprotocol directive and listener middleware plugin type (#1349)

* add support for listener middleware

* add proxyprotocol directive

* make caddy.Listener interface required

* Remove tcpKeepAliveListener wrapper from Serve()
This is now done in the Listen() function, along with other potential middleware.

caddyhttp/httpserver/middleware.go

@@ -6,6 +6,8 @@ import (
 	"os"
 	"path"
 	"time"
+
+	"github.com/mholt/caddy"
 )
 
 func init() {
@@ -18,6 +20,10 @@ type (
 	// passed the next Handler in the chain.
 	Middleware func(Handler) Handler
 
+	// ListenerMiddleware is similar to the Middleware type, except it
+	// chains one net.Listener to the next.
+	ListenerMiddleware func(caddy.Listener) caddy.Listener
+
 	// Handler is like http.Handler except ServeHTTP may return a status
 	// code and/or error.
 	//