Support tables with "rowspan" or "colpsan" attributes

[andrey-skl]

Initial checklist

• I read the support docs
• I read the contributing guide
• I agree to follow the code of conduct
• I searched issues and couldn’t find anything (or linked relevant results below)

Subject

• Have a table with rowspan/colspan and missing cell in next row:

<table>
<tbody>
    <tr>
        <th>Table</th>
        <th>With rowspan</th>
    </tr>
    <tr>
        <td rowspan="2">Rowspan</td>
        <td>test</td>
    </tr>
    <tr>
        <!-- There is no cell because it is occupied by rowspan -->
        <td>test</td>
    </tr>
</tbody>
</table>

Which, when rendered, should look somehow like that:

Problem

When converting to MDAST, the missing cells are added automatically, so the following tree is constructed (notice empty tableCell added to third row's end):

So, if serialize to Markdown, it would look like:

| Table | With rowspan |
| - | - |
| Rowspan | test |
| test | |

Notice the "test" in third column is shifted to the left

Solution

The converter may take "rowspan", "colspan" attributes into account and put empty cells into the corresponding positions, to preserve table structure in general

Alternatives

I don't see any alternative implementation. Related to #54.

[wooorm]

I think that would be a welcome addition! Something in table.js probably, as it has the needed insight on rows and columns!

[wooorm]

Released!