"mingw" top-level field in YAML similar to "nix"

[varosi]

General summary/comments

It would be very useful if there is a "mingw" top-level field in YAML file similar to "nix" where third party C dependencies are listed. Stack may do "pacman -S {dependency}" for each. This way we would have "nix" for Linux and "mingw" for Windows C dependencies.

[snoyberg]

Moved to wishlist https://github.com/commercialhaskell/stack/wiki/Wishlist