I propose to add a functionality to allow Cabal to execute user-defined subcommands like git.
With this, user or library author can add their custom command like cabal doctest etc. (see #2327).

I tried to implement this roughly and basic part is working.
https://github.com/fujimura/cabal/compare/user-defined-subcommands?expand=1

If you can agree with this, I'm happy to finish implementation and make an pull request. Please let me know how do you think and concerns about this. Thanks!