Calendar coloring

- Added 3 new color configuration variables to colorize today, days with due tasks
  and days with overdue tasks in the calendar:
  'calendar.color.today', 'color.calendar.due' and 'calendar.calendar.overdue'

src/tests/cal.t

@@ -30,7 +30,7 @@
 
 use strict;
 use warnings;
-use Test::More tests => 36;
+use Test::More tests => 58;
 
 # Create the rc file.
 if (open my $fh, '>', 'cal.rc')