Добавил SubMux Group

group.go

@@ -0,0 +1,18 @@
+package mux
+
+import (
+	"net/http"
+	"strings"
+)
+
+func Group(mux *http.ServeMux, prefix string, group func(sm *http.ServeMux), middlewares ...Middleware) {
+	groupMux := http.NewServeMux()
+	group(groupMux)
+	if !strings.HasSuffix(prefix, "/") {
+		prefix += "/"
+	}
+	mux.Handle(
+		prefix,
+		http.StripPrefix(strings.TrimSuffix(prefix, "/"), Use(groupMux, middlewares...)),
+	)
+}