[feature] added a completion shell hook for users.

default.nix

@@ -27,6 +27,10 @@ in rec
   };
 
   dev-test = pkgs.stdenv.mkDerivation rec {
+    shellHook = ''
+      source completions/bash/argotk.sh
+      echo "Installed bash completions."
+    '';
     name = "env";
     env = pkgs.buildEnv { name = name; paths = buildInputs; };
     buildInputs = [