docs: def bridge(filename):
      """ Add hash to filename for cache invalidation.
  
      Uses gulp-buster for cache invalidation. Adds current file hash as url arg.
      """
      if not hasattr(settings, 'BASE_DIR'):
          raise Exception("You must provide BASE_DIR in settings for bridge")
      file_path = getattr(settings, 'BUSTERS_FILE', os.path.join('static',
                                                                 'busters.json'))
      buster_file = os.path.join(settings.BASE_DIR, file_path)
      fp = file(buster_file, 'r')
      # TODO: may be store it somewhere to not load file every time
      busters_json = json.loads(fp.read())
      fp.close()
      file_hash = busters_json.get("static/%s" % filename)
      path = static(filename)
      return "%s?%s" % (path, file_hash) if file_hash is not None else path