text: Title: Python function not callable from tavern script for saving 
  Body: Hi, I am calling a save function after getting response from my api, now in the response received i need to format string and save only few elements present,
  ```
         response:
              status_code: 200
              save:
                  headers:
                      res_key:
                          $ext:
                              function: testing_utils:extract_sessid
                              extra_kwargs:
                                  head: headers
  ```
   however, my tavern yaml is unable to call extract_string method in testing_utils file,
   But other functions written in testing_utils are working fine with following syntax
   ```
              verify_response_with:
                  -   function: testing_utils:check_jsonpath_value
  ```
  
  Please help, basically in above way mentioned, the testing_utils file is not accessible ( inside save function) but in same tavern script existing test cases are able to access the same ( with verify_response_with).
                 