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/headlines.html

@@ -1,6 +1,25 @@
 <h1>
-Simple Headline
+Simple Headline <code class="statistic">[1/2]</code>
 </h1>
+<ul>
+<li>
+<p>
+[X] checked
+</p>
+</li>
+<li>
+<p>
+[ ] unchecked
+</p>
+</li>
+<li>
+<p>
+note that statistic tokens are marked up anywhere
+not just where they are actually meant to be - even here &gt; <code class="statistic">[100%]</code> &lt;
+(Org mode proper does the same)
+</p>
+</li>
+</ul>
 <h1>
 <span class="todo">TODO</span>
 <span class="priority">B</span>