Implement the GEDCOM tag "_RUFNAME"

RUFNAME is the German word for "call name", i.e. the part of the given
name commonly used to address somebody. Some official documents mark it
as such.

The GEDCOM standards 5.5.1 and 5.5.5 mention this tag.

gramps/plugins/export/exportgedcom.py

@@ -1304,6 +1304,7 @@ class GedcomWriter(UpdateCallback):
         suffix = name.get_suffix()
         title = name.get_title()
         nick = name.get_nick_name()
+        call = name.get_call_name()
         if nick.strip() == '':
             nick = attr_nick
 
@@ -1323,6 +1324,8 @@ class GedcomWriter(UpdateCallback):
             self._writeln(2, 'SPFX', surprefix)
         if surname:
             self._writeln(2, 'SURN', surname)
+        if call:
+            self._writeln(2, '_RUFNAME', call)
         if name.get_suffix():
             self._writeln(2, 'NSFX', suffix)
         if name.get_title():