Support args::label for showing better descriptions for types in help text

[softdevca]

Fixes #63

[fasterthanlime]

So if you run hooks/install.sh, it'll set up pre-commit and pre-push hooks that would've caught, clippy, docs, and test failures. For this once I'll solve them myself.

[fasterthanlime]

To be even more precise, here is what you should have done, either:

• Checkout main, pull, checkout a fresh branch, cherry-pick the commits you're interested in (without the "doc change to improve --completions help" that I turned down)
• Stay on the same branch, rebase on top of main, git revert the commit I turned down, then git rebase -i $(git merge-base HEAD origin/main) to squash everything in one commit (or just make a note that they should be squashed when accepting the PR).

Again I'm doing the cleanup this time but 1 PR = 1 clean branch = no merge commits. Only rebases.

[softdevca]

Again I'm doing the cleanup this time but 1 PR = 1 clean branch = no merge commits. Only rebases.

Thanks, will do.