add usage information to Command struct

cmd/command.go

@@ -27,6 +27,7 @@ type Command struct {
 	Name  string
 	Func  func(CmdArgs) (int, error)
 	Flags *flag.FlagSet
+	Use   string
 }
 
 var commandRegistry = make(map[string]Command)
@@ -75,6 +76,7 @@ func RegisterCommand(name string, cmd Command) error {
 	if _, ok := commandRegistry[name]; ok {
 		return fmt.Errorf("%s command is already registered")
 	}
+	cmd.Name = name
 	commandRegistry[name] = cmd
 	return nil
 }