Nixify simple_vim idea

I previously had an alias defined as `simple_vim`, which would start an instance
of Vim with a bare bones config. I had a to-do to Nixify it. That is
now (mostly) to-done.

When I try and install it with `nix-env -f ~/briefcase -iA tools.simple_vim`,
Nix fails and says that pkgs.stdenv is undefined. I will need to fix this one
day, but it is neither important nor urgent...

tools/simple_vim/default.nix

@@ -0,0 +1,10 @@
+{ pkgs ? import <nixpkgs> {}, ... }:
+
+let
+  script = pkgs.writeShellScriptBin "simple_vim" ''
+    ${pkgs.vim}/bin/vim -u ${./config.vim}
+  '';
+in pkgs.stdenv.mkDerivation {
+  name = "simple_vim";
+  buildInputs = [ script ];
+}