router.go
func (r *Router) NotFound(handlers ...Handler) {
	handlers = validateAndWrapHandlers(handlers)
	r.notFound = func(rw http.ResponseWriter, req *http.Request) {
		c := r.m.createContext(rw, req)
		c.handlers = make([]Handler, 0, len(r.m.handlers)+len(handlers))
		c.handlers = append(c.handlers, r.m.handlers...)
		c.handlers = append(c.handlers, handlers...)
		c.run()
	}
}
func (r *Router) NotFound(handlers ...Handler) {
	handlers = validateAndWrapHandlers(handlers)
	r.notFound = func(rw http.ResponseWriter, req *http.Request) {
		c := r.m.createContext(rw, req)
		c.handlers = make([]Handler, 0, len(r.m.handlers)+len(handlers))
		c.handlers = append(c.handlers, r.m.handlers...)
		c.handlers = append(c.handlers, handlers...)
		c.run()
	}
}
// NotFound configurates http.HandlerFunc which is called when no matching route is
// found. If it is not set, http.NotFound is used.
// Be sure to set 404 response code in your handler.