Reads the config file from disk or creates a new one.

def _get_config(self):
    '''Reads the config file from disk or creates a new one.'''
    filename = '{}/{}'.format(self.PLUGIN_LOGDIR, CONFIG_FILENAME)
    modified_time = os.path.getmtime(filename)

    if modified_time != self.config_last_modified_time:
      config = read_pickle(filename, default=self.previous_config)
      self.previous_config = config
    else:
      config = self.previous_config

    self.config_last_modified_time = modified_time
    return config