DatesRole: support standard output of date(1).

- Convert to ISO-8601 for the parse.
- Support short month to number in service of above.
- Support TZ to UTC offset in service of above.

In cases where the TZ abbreviation was ambiguous I selected what I
deemed to be the most populous or most likely to be a DDG user's intent.
I may, of course, be wrong.

Fixes #621.