New filter for image paths

Added a new filter fly_attached_file

More info: #33

/**
 * Fly Attached File
 *
 * @param  string $attached_file
 * @param  integer $attachment_id
 * @param  mixed $size
 * @param  boolean $crop
 * @return void
 */
function different_fly_attached_file( $attached_file, $attachment_id, $size, $crop ) {
    return '/path/to/different/file.jpg';
}
add_filter( 'fly_attached_file', 'different_fly_attached_file', 10, 4 );