Add filter for author

On shoretelsky.com we use a modified version of this plugin in order to set a random author for imported posts. By adding a filter for the author, we can use the core plugin without modifications and still have the functionality we need. Example code:

```php
function random_author() {
  $randID = mt_rand(1,100);
  $post_author = '1';
  switch (true) {
    case ($randID <= 25):
      $post_author = '2';
      break;
    case ($randID > 25 AND $randID <= 50):
      $post_author = '3';
      break;
    case ($randID > 50 AND $randID <= 75):
      $post_author = '4';
      break;
    case ($randID > 75 AND $randID <= 100):
      $post_author = '5';
      break;
  }
  return $post_author;
}
add_filter('braftonxml_author', 'random_author');
```