MX::App::Cmd::Simple, and some under-the-hood fixes

ORIGINAL DESCRIPTION: I noticed that this package did not have any support for App::Cmd::Simple, so I added it. I also fixed a few other problems, which are detailed in the commit messages.

This is a copy of a pull request I apparently sent to the wrong repo several years ago: https://github.com/nothingmuch/moosex-app-cmd/pull/1

I've been instructed that this is the proper place to send code. Not surprisingly, my branch from 3 years ago doesn't merge cleanly with the current code. Please let me know if you're still interested in this code and if you'd like me to try and update it, or otherwise how you'd like to proceed here. I haven't looked at the current code, so it's possible lots of the issues fixed in these commits are already addressed.