Option to repair shell configuration files

[CameronBieganek]

As described on Discourse, I made the mistake of overwriting my .zshrc file (on MacOS), after which I could no longer run the julia or juliaup binaries. I fixed the problem by using ./.juliaup/bin/juliaup self uninstall to uninstall juliaup and then reinstalling juliaup. However, @davidanthoff mentioned that it's probably possible to have an option to repair shell configuration files without a complete uninstall and reinstall.