A table with more columns than `columns / 8` is still written as a grid
table, rather than falling back to raw HTML.

```
% pandoc -f html -t markdown-simple_tables-multiline_tables-pipe_tables
<table>
<tr><th>H1</th><th>H2</th><th>H3</th><th>H4</th><th>H5</th><th>H6</th><th>H7</th><th>H8</th><th>H9</th><th>H10</th></tr>
<tr><td>c1</td><td>c2</td><td>c3</td><td>c4</td><td>c5</td><td>c6</td><td>c7</td><td>c8</td><td>c9</td><td>c10</td></tr>
</table>
^D
+----+----+----+----+----+----+----+----+----+-----+
| H1 | H2 | H3 | H4 | H5 | H6 | H7 | H8 | H9 | H10 |
+====+====+====+====+====+====+====+====+====+=====+
| c1 | c2 | c3 | c4 | c5 | c6 | c7 | c8 | c9 | c10 |
+----+----+----+----+----+----+----+----+----+-----+
```

This includes a table with block content in a cell, which has no other
markdown representation.

```
% pandoc -f html -t markdown
<table>
<tr><th>H1</th><th>H2</th><th>H3</th><th>H4</th><th>H5</th><th>H6</th><th>H7</th><th>H8</th><th>H9</th><th>H10</th></tr>
<tr><td><ul><li>a</li><li>b</li></ul></td><td>c2</td><td>c3</td><td>c4</td><td>c5</td><td>c6</td><td>c7</td><td>c8</td><td>c9</td><td>c10</td></tr>
</table>
^D
+------+------+------+------+------+------+------+------+------+------+
| H1   | H2   | H3   | H4   | H5   | H6   | H7   | H8   | H9   | H10  |
+======+======+======+======+======+======+======+======+======+======+
| - a  | c2   | c3   | c4   | c5   | c6   | c7   | c8   | c9   | c10  |
| - b  |      |      |      |      |      |      |      |      |      |
+------+------+------+------+------+------+------+------+------+------+
```