Add option for specifying calendar's date format

Summary:
The new option --dateformat can be used to change the default date format
used by --calendar. The default format 'ddd MMM d yyyy' is no explicitly
stated.

We can't change the default as scripts in the wild probably expect this
format by now. When using locales other than C, 'date' does not understand
this format, though. The new option makes it easy to use e.g. the ISO
format ('yyyy-MM-dd').

BUG: 232712

Reviewers: #kdialog, dfaure

Reviewed By: #kdialog, dfaure

Differential Revision: https://phabricator.kde.org/D29777