Bug Description

This is an enhancement request.

Provide an interface to return a datetime format string one can use with strptime()/strftime() (e.g. "%Y-%m-%d") that the dateutil.parser.parse() uses when parsing a datetime string.

E.g.

>>> dateutil.parser.parse_format('1999-12-09')
'%Y-%m-%d'
>>> dateutil.parser.parse_format('three days ago')
None