if optional positional arg is todo/scheduled/done, then resolve first level

cmd/pttodo-cli/cli.go

@@ -137,7 +137,17 @@ func dump(dry bool, writer io.Writer, filepath string) error {
 	}
 	root.MoveScheduledToTodo()
 
-	b2, err := yaml.Marshal(root)
+	var v interface{} = root
+	switch flag.Arg(0) {
+	case "":
+	case "todo":
+		v = root.Todo
+	case "scheduled":
+		v = root.Scheduled
+	case "done":
+		v = root.Done
+	}
+	b2, err := yaml.Marshal(v)
 	if err != nil {
 		return err
 	}