Add undefined variables feature to example config file.

examples/ansible.cfg

@@ -69,6 +69,10 @@ module_name = command
 , replacing {file}, {host} and {uid} and strftime codes with proper values.
 ansible_managed = Ansible managed: {file} modified on %Y-%m-%d %H:%M:%S by {uid} on {host}
 
+# if set, Ansible will raise errors when attempting to redeference Jinja2
+# variables that are not set in templates or action lines. 
+# error_on_undefined_vars = True
+
 # set plugin path directories here, seperate with colons
 action_plugins     = /usr/share/ansible_plugins/action_plugins
 callback_plugins   = /usr/share/ansible_plugins/callback_plugins