Add nix section to stack.yaml (disabled by default).

This makes it possible to build and install Stack on systems where the
system dependencies (namely zlib) are not present or not installed at
the location GHC/Cabal expects them (e.g NixOS).

stack.yaml

@@ -4,3 +4,9 @@ image:
     base: "fpco/ubuntu-with-libgmp:14.04"
     entrypoints:
       - stack
+
+nix:
+  # --nix on the command-line to enable.
+  enable: false
+  packages:
+  - zlib