milan

Add support for ISO8601 time interval parsing

1

It would be nice to have the parser return a datetime tuple when parsing [ISO intervals](https://en.wikipedia.org/wiki/ISO_8601#Time_intervals) ``` >>> parser.parse('2016-10-04T10:00/2016-10-04T11:15:00') (datetime.datetime(2016, 10, 4, 10, 00), datetime.datetime(2016, 10, 4, 11, 15)) ```