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/HTMLModule/Hypertext.php

@@ -22,6 +22,7 @@ class HTMLPurifier_HTMLModule_Hypertext extends HTMLPurifier_HTMLModule
                 // 'type' => 'ContentType',
             )
         );
+        $a->formatting = true;
         $a->excludes = array('a' => true);
     }