goctl add stdin flag (#170)

* add stdin flag to use stdin receive api doc and use stdout output formatted result

* optimize code and output error through stderr

* fix mistake

* add dir parameter legality verify

tools/goctl/api/gogen/gen.go

@@ -66,7 +66,7 @@ func DoGenProject(apiFile, dir string, force bool) error {
 		return err
 	}
 
-	if err = apiformat.ApiFormat(apiFile, false); err != nil {
+	if err := apiformat.ApiFormatByPath(apiFile); err != nil {
 		return err
 	}