Pass $user_id to ass_digest_format_item_group filter.

See #120.

bp-activity-subscription-digest.php

@@ -463,14 +463,15 @@ function ass_digest_format_item_group( $group_id, $activity_ids, $type, $group_n
 	/**
 	 * Filters the markup for a group's digest section.
 	 *
-	 * @since 3.8.0 Introduced $activity_ids parameter.
+	 * @since 3.8.0 Introduced $activity_ids and $user_id parameters.
 	 *
 	 * @param string $group_message Markup.
 	 * @param int    $group_id      ID of the group.
 	 * @param string $type          Digest type. 'dig' or 'sum'.
 	 * @param array  $activity_ids  Array of IDs included in this group's digest.
+	 * @param int    $user_id       ID of the user receiving the digest.
 	 */
-	return apply_filters( 'ass_digest_format_item_group', $group_message, $group_id, $type, $activity_ids );
+	return apply_filters( 'ass_digest_format_item_group', $group_message, $group_id, $type, $activity_ids, $user_id );
 }