watson: added bash/zsh completions

pkgs/applications/office/watson/default.nix

@@ -1,3 +1,4 @@
+{ stdenv, pythonPackages, fetchpatch, installShellFiles }:
 
 with pythonPackages;
 
@@ -15,8 +16,14 @@ buildPythonApplication rec {
     pytest -vs tests
   '';
 
+  postInstall = ''
+    installShellCompletion --bash --name watson watson.completion
+    installShellCompletion --zsh --name _watson watson.zsh-completion
+  '';
+
   checkInputs = [ py pytest pytest-datafiles mock pytest-mock pytestrunner ];
   propagatedBuildInputs = [ requests click arrow ];
+  nativeBuildInputs = [ installShellFiles ];
 
   meta = with stdenv.lib; {
     homepage = "https://tailordev.github.io/Watson/";