Default command with parameters

Hi,

I use the `default_command` argument of `Manager.run` to define `runserver` as default command: `manager.run(default_command="runserver")`. It works well but it isn't possible to specify arguments for the default command.

```
$ flask-app --port 8080
usage: flask-app [-?] {runserver} ...
daas-backend: error: invalid choice: '8080' (choose from 'runserver')
```

Regards,
Christoph
