Optionally use haskell packages from nix.

It's a feature request.
Is it possible to use stack as a thin frontend to nix, and install even haskell dependencies from here?

It may have format like

```
nix: true
sys: true
```

or

```
nix: true
resolver: nix-ghc-7.10.3
```

Also, in this case `nix` should understand `extra-dependencies` from `git`.