Better support for custom formats

This patch allows other modules to define custom formats using hook_mailing_label_formats()

Example:

/**
 * Implementation of hook_mailing_label_formats()
 */
function mymodule_mailing_label_formats() {
  return array( 
    'mymodule-123' => array(
      'name' => 'My Module 123',
      'paper-size' => 'A4', 
      'metric' => 'mm', 
      'lMargin' => 6,
      'tMargin' => 15.1, 
      'NX' => 3, 
      'NY' => 7, 
      'SpaceX' => 2.5, 
      'SpaceY' => 0,
      'width' => 63.5, 
      'height' => 38.1, 
      'font-size' => 9,
    ),
  );
}