Serving index file when route not found under Negroni

I am using Golang, Negroni, and Gorilla mux for a web api server. I have my api routes under /api and I am using Negroni to serve static files from my /public directory using urls under /. I would like to serve my index.html file (containing a single page javascript application) not only if it is requested by name or as the index file, but also if the request would otherwise result in a 404 because it doesn't correspond to a route or a file in the /public directory. This is so that those URLs will load the webapp which will transition to the correct route (client side javascript history/pushState) or else give a not found error if that resource doesn't exist. Is there a way to get Negroni's static middleware or Gorilla mux to do this?

The Router type in the mux library has a NotFoundHandler field of type http.Handler. This would allow you to handle a unmatched route as you see fit:

// NotFoundHandler overrides the default not found handler
func NotFoundHandler(w http.ResponseWriter, r *http.Request) {
    // You can use the serve file helper to respond to 404 with
    // your request file.

    http.ServeFile(w, r, "public/index.html")
}

func main() {
    r := mux.NewRouter()
    r.NotFoundHandler = http.HandlerFunc(NotFoundHandler)

    // Register other routes or setup negroni

    log.Fatal(http.ListenAndServe(":8080", r))
}