add support for custom MethodNotAllowed function

router.go

diff --git a/router.go b/router.go
index 4893950..8c9550f 100644
--- a/router.go
+++ b/router.go
@@ -142,6 +142,11 @@
 	// found. If it is not set, http.NotFound is used.
 	NotFound http.HandlerFunc
 
+	// Configurable http.HandlerFunc which is called when a request
+	// cannot be routed and HandleMethodNotAllowed is true.
+	// If it is not set, http.Error with http.StatusMethodNotAllowed is used.
+	MethodNotAllowed http.HandlerFunc
+
 	// Function to handle panics recovered from http handlers.
 	// It should be used to generate a error page and return the http error code
 	// 500 (Internal Server Error).
@@ -335,10 +340,14 @@
 
 			handle, _, _ := r.trees[method].getValue(req.URL.Path)
 			if handle != nil {
-				http.Error(w,
-					http.StatusText(http.StatusMethodNotAllowed),
-					http.StatusMethodNotAllowed,
-				)
+				if r.MethodNotAllowed != nil {
+					r.MethodNotAllowed(w, req)
+				} else {
+					http.Error(w,
+						http.StatusText(http.StatusMethodNotAllowed),
+						http.StatusMethodNotAllowed,
+					)
+				}
 				return
 			}
 		}

router_test.go

diff --git a/router_test.go b/router_test.go
index 6292ba8..e5f3e09 100644
--- a/router_test.go
+++ b/router_test.go
@@ -178,6 +178,20 @@
 	if !(w.Code == http.StatusMethodNotAllowed) {
 		t.Errorf("NotAllowed handling route %s failed: Code=%d, Header=%v", w.Code, w.Header())
 	}
+
+	w = httptest.NewRecorder()
+	responseText := "custom method"
+	router.MethodNotAllowed = func(w http.ResponseWriter, req *http.Request) {
+		w.WriteHeader(http.StatusTeapot)
+		w.Write([]byte(responseText))
+	}
+	router.ServeHTTP(w, r)
+	if got := w.Body.String(); !(got == responseText) {
+		t.Errorf("unexpected response got %q want %q", got, responseText)
+	}
+	if w.Code != http.StatusTeapot {
+		t.Errorf("unexpected response code %d want %d", w.Code, http.StatusTeapot)
+	}
 }
 
 func TestRouterNotFound(t *testing.T) {