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/math_usage/document.md

@@ -0,0 +1,13 @@
+css: math.css
+
+This is an example document. Inline: $a$, $b$, $q$ and stop.
+
+$$        \int_a^b f(x) dx       $$     (1)
+
+See for example (eq:1).
+
+Equation inline: $  \int_a^b f(x) dx $ and that's it.
+
+
+Spacing: Maruku outputs sizes and alignment for images in `ex`. An `ex` is one `x`:
+x, $\textrm{x}$, $x$ should have the same height.