Make heading in appointments panel configurable

Add a new configuration variable format.dayheading to set the format of
the date displayed at the top of the event and appointment list.

Signed-off-by: Lars Henriksen <LarsHenriksen@get2net.dk>
Signed-off-by: Lukas Fleischer <lfleischer@calcurse.org>

src/vars.c

@@ -147,6 +147,7 @@ void vars_init(void)
 	conf.progress_bar = 1;
 	strncpy(conf.output_datefmt, "%D", 3);
 	conf.input_datefmt = 1;
+	strcpy(conf.day_heading, DAY_HEADING_DEFAULT);
 
 	datefmt_str[0] = _("mm/dd/yyyy");
 	datefmt_str[1] = _("dd/mm/yyyy");