Displaying instructions when installing executables through cabal-install without the need for a Custom build type

I was checking out the `ghc-heap-view` package and I discovered that @nomeata had a clever use of the `Custom` build type to display instructions for the program: https://github.com/nomeata/ghc-heap-view/blob/master/Setup.hs

Of course this is very helpful because `cabal-install` is today the main installation vector for Haskell binaries, but also this doesn't help us as Custom build types are the Devil.

My very rough impression is that we could expose a way to print messages upon installation, with a variety of pre-computed, immutable values exposed, like the data dir, the flags, etc.

What do you think about this?

Edit: I'm not advocating for a generic post-install hook mechanism inside the "Simple" build type, because it's good to have restricted capabilities in terms of code execution, and expose more "focused" features.