Easily to start locally

[suntong]

Solving #23's problem slightly differently -- since algnhsa wraps normal go http server to use with as lambda, it'd be better to allow people to start their normal go http server locally without the Lambda adapter / wrapper.

The idea is borrowed from here --
https://blog.carlmjohnson.net/post/2020/how-to-host-golang-on-netlify-for-free/

func main() {
    port := flag.Int("port", -1, "specify a port to use http rather than AWS Lambda")
    flag.Parse()
    listener := gateway.ListenAndServe
    portStr := "n/a"
    if *port != -1 {
        portStr = fmt.Sprintf(":%d", *port)
        listener = http.ListenAndServe
        http.Handle("/", http.FileServer(http.Dir("./public")))
    }

    http.Handle("/api/feed", feed2json.Handler(
        feed2json.StaticURLInjector("https://news.ycombinator.com/rss"), nil, nil, nil))
    log.Fatal(listener(portStr, nil))
}

With this version of the code, if we run go run main.go -port 8000, the server will start on http://localhost:8000, but if we omit a port, it will start in AWS Lambda mode. It also runs a file server in HTTP mode, so we can preview the static files that we’re working on in our browser as we’re developing.

I realized that doing this way will have API changes for algnhsa, but maybe we can use a different function name or something (say algnhsa.CompatibleListenAndServe), as this seamless switching feature is really neat IMHO.

Please consider. Thanks