requiredOption make `--help` invalid

when i set requiredOption on program, the `--help` will noneffective:
```js
const program = new commander.Command()
program.version('0.0.1')
program
  .requiredOption('-d, --dir <path>', 'dir path')
  .option('-t, --timeout <number>', 'timeout')
```
when use -h, will get a error, but not the help info:
```shell
$ ./app.js -h
error: required option '-d, --dir <path>' not specified
```