httpserver: Add grpc plugin directive (#1694)

* Add grpc plugin directive

* Update plugin.go

Removed whitespace line

* Update plugin.go

caddyhttp/httpserver/plugin.go

@@ -490,6 +490,7 @@ var directives = []string{
 	"hugo",      // github.com/hacdias/caddy-hugo
 	"mailout",   // github.com/SchumacherFM/mailout
 	"awslambda", // github.com/coopernurse/caddy-awslambda
+	"grpc",      // github.com/pieterlouw/caddy-grpc
 }
 
 const (