Add HTMLPurifier to extlib

HTMLPurifier defangs arbitrary submitted HTML. We're using it in the
OStatus plugin, but it may be valuable for other parts of the codebase
(I think OEmbed might benefit, for example).

extlib/HTMLPurifier/HTMLPurifier/VarParserException.php

@@ -0,0 +1,11 @@
+<?php
+
+/**
+ * Exception type for HTMLPurifier_VarParser
+ */
+class HTMLPurifier_VarParserException extends HTMLPurifier_Exception
+{
+
+}
+
+// vim: et sw=4 sts=4