Add basic support for statistic tokens (e.g. [100%] [1/1])

Org mode does not care where those tokens are when it comes to the
export (afaict). We'll do the same.

(They should only be in the first line of a list item or a headline)

org/testdata/misc.html

@@ -43,7 +43,12 @@ src block
 </p>
 <div class="highlight">
 <pre>
-* Simple Headline
+* Simple Headline [1/2]
+- [X] checked
+- [ ] unchecked
+- note that statistic tokens are marked up anywhere
+  not just where they are actually meant to be - even here &gt; [100%] &lt;
+  (Org mode proper does the same)
 * TODO [#B] Headline with todo status &amp; priority
 * DONE Headline with TODO status
 :PROPERTIES: