Add location-separator field

Analogous to `description-separator` and `url-separator` this can be
used in a format string when the `location` information is desired.

Author: Mark Knoop

doc/source/usage.rst

@@ -118,6 +118,9 @@ Several options are common to almost all of :program:`khal`'s commands
    location
         The event location.
 
+   location-separator
+        A separator: " @ " that appears when there is a location.
+
    calendar
         The calendar name.
 

khal/khalendar/event.py

@@ -738,6 +738,9 @@ def fmt(s: str) -> str: return s.strip()
         if attributes["description"]:
             attributes["description-separator"] = " :: "
         attributes["location"] = self.location.strip()
+        attributes["location-separator"] = ""
+        if attributes["location"]:
+            attributes["location-separator"] = " @ "
         attributes["attendees"] = self.attendees
         attributes["all-day"] = str(allday)
         attributes["categories"] = self.categories