Add invoke tab-completion script to dev .bashrc

This enables tab-completion for the new invoke tasks in the dev version.
So for example, you can type "invoke ty<Tab>" and it will complete to
"type-checking".

salt/salt/development.sls

@@ -19,3 +19,9 @@ automatic-activate:
   file.append:
     - name: '/home/{{ app_username }}/.bashrc'
     - text: 'source activate'
+
+# adds invoke's tab-completion script so that invoke tasks can be completed
+invoke-tab-completion:
+  file.append:
+    - name: '/home/{{ app_username }}/.bashrc'
+    - text: 'source <(invoke --print-completion-script bash)'