Support for interval-shorthand

[7yl4r]

This is a feature request for interval shorthand as described by the standard:

For / expressions, if any elements are missing from the end value, they are assumed to be the same as for the start value including the time zone. This feature of the standard allows for concise representations of time intervals. For example, the date of a two-hour meeting including the start and finish times could be simply shown as "2007-12-14T13:30/15:30", where "/15:30" implies "/2007-12-14T15:30" (the same date as the start), or the beginning and end dates of a monthly billing period as "2008-02-15/03-14", where "/03-14" implies "/2008-03-14" (the same year as the start).

Input:

from iso8601utils import parsers
parsers.interval('2016-02-17T01:00/02:01')

Expected:

iso8601utils.interval(2016-02-17T01:00:00.000000Z/2016-02-17T02:01:00.000000Z)

Observed:

parsers.interval(2016-02-17T01:00/02:01)
                                 ^
SyntaxError: invalid syntax