add 'current-date' attribute to events

When printing multi-day events through 'khal list --json', each day gets
its own entry. However there is no indication which event belongs to
which day. The user would manually need to recalculate it.

Add a new attribute which references the specific day of a multi-day
event that is printed.

Author: t-8ch

khal/khalendar/event.py

@@ -645,6 +645,8 @@ def attributes(
         attributes["end-date-long"] = self.end_local.strftime(self._locale['longdateformat'])
         attributes["end-time"] = self.end_local.strftime(self._locale['timeformat'])
 
+        attributes["current-date"] = relative_to_start.strftime(self._locale['dateformat'])
+
         attributes["duration"] = timedelta2str(self.duration)
 
         # should only have time attributes at this point (start/end)

khal/utils.py

@@ -221,7 +221,7 @@ def fmt(rows):
                       'start-style', 'end-style', 'to-style', 'start-end-time-style',
                       'end-necessary', 'end-necessary-long', 'repeat-symbol', 'repeat-pattern',
                       'title', 'organizer', 'description', 'location', 'all-day', 'categories',
-                      'uid', 'url', 'calendar', 'calendar-color', 'status', 'cancelled']
+                      'uid', 'url', 'calendar', 'calendar-color', 'status', 'cancelled', 'current-date']
 
 
 def json_formatter(fields):