Add support for list item checkboxes (e.g. [X])

see https://orgmode.org/manual/Checkboxes.html
We're deviating from Org mode regarding the assigned css classes but the chosen
classes feel better than (on, off, trans) and it's not like the export matches
1:1 otherwise.

org/testdata/headlines.html

@@ -2,14 +2,14 @@
 Simple Headline <code class="statistic">[1/2]</code>
 </h1>
 <ul>
-<li>
+<li class="checked">
 <p>
-[X] checked
+checked
 </p>
 </li>
-<li>
+<li class="unchecked">
 <p>
-[ ] unchecked
+unchecked
 </p>
 </li>
 <li>