Detect if domxml extension is loaded, and use DirectLex accordingly.

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

tests/HTMLPurifier/PHPT/domxml.phpt

@@ -0,0 +1,15 @@
+--TEST--
+DirectLex with domxml test
+--SKIPIF--
+<?php
+if (!extension_loaded('dom')) {
+    echo "skip - dom not available";
+} elseif (!extension_loaded('domxml')) {
+    echo "skip - domxml not loaded";
+}
+--FILE--
+<?php
+require '../library/HTMLPurifier.auto.php';
+echo get_class(HTMLPurifier_Lexer::create(HTMLPurifier_Config::createDefault()));
+--EXPECT--
+HTMLPurifier_Lexer_DirectLex