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.path.php

@@ -0,0 +1,11 @@
+<?php
+
+/**
+ * @file
+ * Convenience stub file that adds HTML Purifier's library file to the path
+ * without any other side-effects.
+ */
+
+set_include_path(dirname(__FILE__) . PATH_SEPARATOR . get_include_path() );
+
+// vim: et sw=4 sts=4