Store "rebar" in app.extensions on rebar.init_app (plangrid#230)

Include handler_registries so that it's possible to enumerate all the
Rebar APIs a Flask app provides given only a reference to the app object.

flask_rebar/rebar.py

@@ -791,6 +791,8 @@ def init_app(self, app):
         for registry in self.handler_registries:
             registry.register(app=app)
 
+        app.extensions["rebar"] = {"handler_registries": self.handler_registries}
+
     def _init_error_handling(self, app):
         @app.errorhandler(errors.HttpJsonError)
         def handle_http_error(error):