Bugfix: datefmt on setFormatter

[tombo315]

Repro:

>>> from rainbow_logging_handler import *
>>> DATE_FORMAT_STRING = '%m/%d/%Y %I:%M:%S %p'
>>> from logging import *
>>> f = Formatter('[%(asctime)s] %(name)s %(funcName)s():%(lineno)d\t%(message)s', datefmt=DATE_FORMAT_STRING)
>>> h = RainbowLoggingHandler(sys.stdout)
>>> h.setFormatter(f)
>>> l.addHandler(h)
>>> l.info('no date!')
[12:45:50] root <module>():1    no date!