Implement "carryover" functionality, requested by Kinderlehrer <bitweaver@7doves.com>

This commit is a limited implementation of the "active formatting
elements" algorithm implemented in HTML5, which preserves certain
formatting elements such as <a> and <b> when exiting or entering nodes.

Signed-off-by: Edward Z. Yang <edwardzyang@thewritingpot.com>

library/HTMLPurifier/Token.php

@@ -19,6 +19,7 @@ class HTMLPurifier_Token {
      */
     public $skip;
     public $rewind;
+    public $carryover;
 
     public function __get($n) {
       if ($n === 'type') {