Mediawiki reader: improve table parsing.

This fixes detection of table attributes and also
handles `!` characters in cells.

Closes #4508.

test/command/2649.md

@@ -67,7 +67,8 @@
 ! Aantal
 |-
 | 1
-|align=left| {{FR-VLAG}} [[Sébastien Loeb]] | 78
+|align=left| {{FR-VLAG}} [[Sébastien Loeb]]
+| 78
 |-
 | 2
 |align=left| {{FR-VLAG}} '''[[Sébastien Ogier]]'''
@@ -79,24 +80,25 @@
 |}
 ^D
 <table>
+<thead>
+<tr class="header">
+<th><p>Plaats</p></th>
+<th><p>Rijder</p></th>
+<th><p>Aantal</p></th>
+</tr>
+</thead>
 <tbody>
 <tr class="odd">
-<td></td>
-<td><p>Plaats</p></td>
-<td><p>Rijder</p></td>
-<td><p>Aantal</p></td>
-</tr>
-<tr class="even">
 <td><p>1</p></td>
 <td><p><a href="Sébastien_Loeb" title="wikilink">Sébastien Loeb</a></p></td>
 <td><p>78</p></td>
 </tr>
-<tr class="odd">
+<tr class="even">
 <td><p>2</p></td>
 <td><p><strong><a href="Sébastien_Ogier" title="wikilink">Sébastien Ogier</a></strong></p></td>
 <td><p>38</p></td>
 </tr>
-<tr class="even">
+<tr class="odd">
 <td><p>10</p></td>
 <td><p><a href="Hannu_Mikkola" title="wikilink">Hannu Mikkola</a></p></td>
 <td><p>18</p></td>