Feature request: allow it to accept no datetime and default to utcnow

[gdalmau]

Description

It would be good to be able to use this context manager/decorator as a way to tell the tests to just freeze time without having to specify any datetime.

Currently it accepts None as a possible value and it defaults to .utcnow(), so it looks like the implementation should be trivial:

freezegun/freezegun/api.py

Lines 460 to 465 in 33fdce8

	def _parse_time_to_freeze(time_to_freeze_str):
	"""Parses all the possible inputs for freeze_time
	:returns: a naive ``datetime.datetime`` object
	"""
	if time_to_freeze_str is None:
	time_to_freeze_str = datetime.datetime.utcnow()