Use keyword to set positional argument

I don't think this issue has been submitted before, but #56 seems to want the same sort of thing.

I want this syntax to be the normal usage:

app inputfile outputfile

But I want both arguments to be optional and fallback on defaults:

app

Which means that it would be nice to specify one but not the other:

app inputfile
app -t outputfile

Therefore, I'd like to be able to set outputfile via *either* a positional or a keyword argument. Is this supported?