Make it the verbose log.

        A verbose log can be only shown when user want to see more logs.
        It works as::

            log.verbose.warn('this is a verbose warn')
            log.verbose.info('this is a verbose info')

def verbose(self):
        """
        Make it the verbose log.

        A verbose log can be only shown when user want to see more logs.
        It works as::

            log.verbose.warn('this is a verbose warn')
            log.verbose.info('this is a verbose info')
        """

        log = copy.copy(self)
        log._is_verbose = True
        return log