Update to latest HTML5lib, Add Maruku testdir

Sync with the latest html5lib.
Having the Maruku unit tests on-hand may be useful for debugging; so let's include them.

vendor/plugins/maruku/tests/unittest/xml2.md

@@ -0,0 +1,34 @@
+Write a comment here
+*** Parameters: ***
+{}
+*** Markdown input: ***
+<!--
+<
+-->
+*** Output of inspect ***
+md_el(:document,[md_html("<!--\n<\n-->")],{},[])
+*** Output of to_html ***
+<!--
+<
+-->
+*** Output of to_latex ***
+
+*** Output of to_md ***
+
+*** Output of to_s ***
+
+*** EOF ***
+
+
+
+	OK!
+
+
+
+*** Output of Markdown.pl ***
+<!--
+<
+-->
+
+*** Output of Markdown.pl (parsed) ***
+Error: #<NoMethodError: private method `write_children' called for <div> ... </>:REXML::Element>