Optionally create wrappers for ghc programs on setup

When a user uses stack to manage GHC installations they can call ghc programs (such as `ghc`, `ghci`, `runghc`, etc) directly by running for example `stack exec -- ghc Main.hs`. However, many tutorials and tools expect `ghc` and friends to be available on path.

In order to have `ghc` related programs available on path one can:

1. Install GHC separately
2. Create a wrapper shell program that runs `stack exec -- ghci $@`

I use (2) and it works pretty well, so my suggestion is to streamline (2) as a feature in stack: when running `stack setup`, stack could check if the ghc programs are available on path, and if they aren't suggest to create wrapper scripts for the user (in `~/.local/bin` for example).

The benefit is that users which are not interested in installing ghc manually can use tutorials and tools that expect ghc programs on path without modification.