vendor: codegangsta/cli to urfave/cli

For https://github.com/coreos/etcd/issues/3901.

cmd/vendor/github.com/urfave/cli/cli.go

@@ -0,0 +1,19 @@
+// Package cli provides a minimal framework for creating and organizing command line
+// Go applications. cli is designed to be easy to understand and write, the most simple
+// cli application can be written as follows:
+//   func main() {
+//     cli.NewApp().Run(os.Args)
+//   }
+//
+// Of course this application does not do much, so let's make this an actual application:
+//   func main() {
+//     app := cli.NewApp()
+//     app.Name = "greet"
+//     app.Usage = "say a greeting"
+//     app.Action = func(c *cli.Context) error {
+//       println("Greetings")
+//     }
+//
+//     app.Run(os.Args)
+//   }
+package cli