Add table pretty printing & alignment

Also dismissed implementing colgroups for now - had it but didn't like the
added complexity for a very questionable benefit - i've actually never used
that feature of org tables...

org/testdata/lists.html

@@ -60,11 +60,17 @@ and another one with a table
 </p>
 <table>
 <thead>
-<th>a</th><th>b</th><th>c</th>
+<tr>
+<th class="align-right">a</th>
+<th class="align-right">b</th>
+<th class="align-right">c</th>
+</tr>
 </thead>
 <tbody>
 <tr>
-<td>1</td><td>2</td><td>3</td>
+<td class="align-right">1</td>
+<td class="align-right">2</td>
+<td class="align-right">3</td>
 </tr>
 </tbody>
 </table>