pipxrc to improve upgrade, reinstall, reinstall, reinstall-all behaviors

[itsayellow]

I'd like to try and code up the idea @cs01 talked about in Issue #75 . The idea would be a small file in each venv called 'pipxrc' or similar, that contained at least the spec for the package. Also it could include other useful things like injected packages.

How would this feature be useful?
It would make reinstall and reinstall-all be able to actually reinstall the exact same package(s), even if they were not found on pypi (or worse, had the same name as an unrelated pypi package). It would make sure upgrade with no arguments upgraded the proper packages. And it could also re-install injected packages after a reinstall. It would solve a lot of problems, e.g. likely #63, #87, #75. It would also allow a way to allow for a system-upgraded python to not break the venvs (#146) by enabling reinstall-all to recreate the venvs.

Describe the solution you'd like
pipxrc file. I'm willing to code it.

Describe alternatives you've considered

I open this ticket because I'd like to help out, but am just starting to look at the pipx code. If there are some hints as to code organization that would mesh well with pipx, I'd be happy to hear them (especially from @cs01). Otherwise I will start modifying my fork and post a pull request.

[itsayellow]

Fixed via #222