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/alt.md

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+Write a comment here
+*** Parameters: ***
+{} # params 
+*** Markdown input: ***
+ ![bar](/foo.jpg)
+
+
+*** Output of inspect ***
+md_el(:document,[md_par([md_im_image(["bar"], "/foo.jpg", nil)])],{},[])
+*** Output of to_html ***
+<p><img src='/foo.jpg' alt='bar' /></p>
+*** Output of to_latex ***
+
+*** Output of to_md ***
+bar
+*** Output of to_s ***
+bar
+*** EOF ***
+
+
+
+	OK!
+
+
+
+*** Output of Markdown.pl ***
+<p><img src="/foo.jpg" alt="bar" title="" /></p>
+
+*** Output of Markdown.pl (parsed) ***
+Error: #<NoMethodError: private method `write_children' called for <div> ... </>:REXML::Element>