Usage needs to show argument placeholders

[lukehutch]

Currently there is no distinction in the usage list between boolean type parameters (which are just either present or absent, and don't take an argument), and parameters that require an argument (e.g. parameters that have type String).

What I would expect is something along the lines of not

  Options:
    -h, --help, -?
      Print this help message and exit
    -i, --input
      The input file

but rather

  Options:
    -h, --help, -?
      Print this help message and exit
    -i, --input <input_file>
      The input file

In other words I would like to be able to provide an extra argument placeholder="<input_file>" to the @Parameter annotation that is displayed in the above way.

I know there are custom usage formatters, but the placeholder would still need to be given in the annotation, so this really should be supported out of the box.

[mkarg]

As long as it contains a test proving that existing applications will not fail but new applications will show this placeholder, I would be delightet to review your PR. 👍

[lukehutch]

I submitted a PR. I did not include updates to the documentation to the PR (that would need to be done too).