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?

先阅读相关 issue #56 以及命令行参数解析的入口点，以确定当前是否可以通过任一语法提供值。重现 issue 中的三种调用形式，然后确定位置参数和关键字参数是否可以共享同一个目标，以及默认值的行为，再实现或记录结果。

由索引模型根据 Issue 内容生成。