New DirOptions.Cache field for in-memory caching and pre-compression for the fastest possible static file server

Read HISTORY.md it contains a breaking change, second parameter of HandleDir should be iris.Dir(...) instead of just a string

relative to: https://github.com/kataras/iris/issues/1556#issuecomment-661057446


Former-commit-id: 14b48a06fb3b99287dff543932be2937a64233b9

_examples/websocket/socketio/main.go

@@ -46,7 +46,7 @@ func main() {
 	defer server.Close()
 
 	app.HandleMany("GET POST", "/socket.io/{any:path}", iris.FromStd(server))
-	app.HandleDir("/", "./asset")
+	app.HandleDir("/", iris.Dir("./asset"))
 
 	app.Listen(":8000", iris.WithoutPathCorrection)
 }