Recurring Tasks - new "weekdays" frequency

- Added support for "weekdays" as a recurrence frequency, which skips
  Saturday and Sunday, but is otherwise a daily recurrence.  Thanks
  to Chris Pride.

src/tests/recur.t

@@ -54,9 +54,6 @@ $output = qx{../task rc:recur.rc desc};
 like ($output, qr/second .* third .* first/msx, 'weekly 3d daily');
 
 # Cleanup.
-unlink 'shadow.txt';
-ok (!-r 'shadow.txt', 'Removed shadow.txt');
-
 unlink 'pending.data';
 ok (!-r 'pending.data', 'Removed pending.data');