Add hide check to write_arg_markdown

[wackazong]

Hidden arguments are now correctly filtered out from the markdown documentation.

[ilyagr]

This PR includes a lot of unrelated changes. I think only the snippet

    // Don't document commands marked with `clap(hide = true)`.
    if arg.is_hide_set() {
        return Ok(());
    }

is relevant.

I am not the maintainer, but I think that, to make this PR mergeable, you should get rid of all the unrelated stuff and change one of the tests to have a hidden argument.

(I do like many of your unrelated changes, but you could make them a separate draft PR or you could post a link to your fork with those changes).

@ConnorGray , I'm happy to create a cleaner PR with a test myself, if it helps and if you are planning/able to release any future versions of this library. (Also, thanks for writing this library!)

[wackazong]

@ilyagr your are absolutely right, I accidentally pushed more changes to this PR. Only the first commit is relevant. And I agree with you regarding the test as well. Thanks for the input.

[ilyagr]

I put a cleaned-up version of this with tests in #22, specifically 48907e1. My approach is a little different, note the test of a command that only has hidden options.

[ConnorGray]

@wackazong @ilyagr Thank you both for your contributions to this library, and please accept my apologies for the delay in processing PRs in this repo😌

I agree with @ilyagr this is a bit large to merge in one go, but I've merged the piece Ilya broke out in #22; if there are other parts of this PR you'd like to see get merged, I'm happy to discuss them in a new PR 🙂