Use filename completion after first argument

[ericsong]

Addressing - #4 (comment)

[dmohs]

I don't think a generic solution is appropriate for this. While this works for a common case, one would expect to want to use this framework in a command group scenario, where filename completion would only be appropriate two or three levels in and would otherwise be confusing.

My preferred approach is to give the script writer control over where filename completion happens by making it an explicit option during definition, then providing the list by-hand as you've done here (via ls).