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/goctl.go

@@ -51,15 +51,16 @@ var (
 							Name:  "dir",
 							Usage: "the format target dir",
 						},
-						cli.BoolFlag{
-							Name:  "p",
-							Usage: "print result to console",
-						},
 						cli.BoolFlag{
 							Name:     "iu",
 							Usage:    "ignore update",
 							Required: false,
 						},
+						cli.BoolFlag{
+							Name:     "stdin",
+							Usage:    "use stdin to input api doc content, press \"ctrl + d\" to send EOF",
+							Required: false,
+						},
 					},
 					Action: format.GoFormatApi,
 				},