Add --go-module-name flag to support generating Go module compatible code (#7651)

* Add --go-module-name flag to support generating code for go modules

* Rename echo example folder

* Grammar

* Update readme for go-echo example

* Update readme for go-echo example

* Re-enable go modules after test is done

examples/go-echo/server/server.go

@@ -0,0 +1,35 @@
+package main
+
+import (
+	"echo/net"
+	"fmt"
+	"io/ioutil"
+	"net/http"
+
+	flatbuffers "github.com/google/flatbuffers/go"
+)
+
+func echo(w http.ResponseWriter, r *http.Request) {
+	body, err := ioutil.ReadAll(r.Body)
+	if err != nil {
+		fmt.Printf("Unable to read request body: %v\n", err)
+		return
+	}
+
+	// Last 4 bytes is offset. See client.go.
+	off := flatbuffers.GetUOffsetT(body[len(body)-4:])
+	buf := body[:len(body) - 4] 
+
+	req := net.GetRootAsRequest(buf, off)
+	player := req.Player(nil)
+
+	fmt.Printf("Got request (name: %v, hp: %v)\n", string(player.Name()), player.Hp())
+	w.Write(body)
+}
+
+func main() {
+	http.HandleFunc("/echo", echo)
+
+	fmt.Println("Listening on port :8080")
+	http.ListenAndServe(":8080", nil)
+}