Fix headline tags, table pretty printing and multiline links

- we can't just look at the len of the string (~ #bytes) - that breaks down for
  tables containing characters consisting of multiple bytes. This handles
  more (still not all) cases and is good enough for now
- add _ to allowed tag chars - also require space between headline and tags
- links (link itself, not the description) spanning multiple lines are not
  supported - otherwise we would have to take care of splitting link and adding
  indentation for org pretty printing - and that sounds like such an edge case
  that it seems cleaner to forbid them

org/testdata/tables.html

@@ -42,6 +42,42 @@ table with separator after header
 </figure>
 <figure>
 <table>
+<thead>
+<tr>
+<th>Character</th>
+<th>Org</th>
+<th>Rendered HTML</th>
+</tr>
+</thead>
+<tbody>
+<tr>
+<td>Hyphen</td>
+<td><code class="verbatim">a - b</code></td>
+<td>a - b</td>
+</tr>
+<tr>
+<td>Ndash</td>
+<td><code class="verbatim">a -- b</code></td>
+<td>a – b</td>
+</tr>
+<tr>
+<td>Mdash</td>
+<td><code class="verbatim">a --- b</code></td>
+<td>a — b</td>
+</tr>
+<tr>
+<td>Ellipsis</td>
+<td><code class="verbatim">a ... b</code></td>
+<td>a … b</td>
+</tr>
+</tbody>
+</table>
+<figcaption>
+table with unicode characters
+</figcaption>
+</figure>
+<figure>
+<table>
 <tbody>
 <tr>
 <td class="align-right">1</td>