Add hook for custom jupyterhub_config.py content

This lets extensions directly control how JupyterHub
is configured

integration-tests/plugins/simplest/tljh_simplest.py

@@ -35,4 +35,8 @@ def tljh_config_post_install(config):
     # Put an arbitrary marker we can test for
     config['simplest_plugin'] = {
         'present': True
-    }
+    }
+
+@hookimpl
+def tljh_custom_jupyterhub_config(c):
+    c.JupyterHub.authenticator_class = 'tmpauthenticator.TmpAuthenticator'