Add support for offset column data group

[duboisp]

Cons	Subject	Pros
bad	Idea	good
No	Question	Yes
Negative	Pole	Positive

The table above have 3 colgroup, two data column group and one header column group.

It could be coded like the following

```xml
<table>
    <caption>Offset data column group</caption>
    <!-- Data Column Group -->
    <colgroup>
        <col />
    </colgroup>
    <!-- Header Column Group -->
    <colgroup>
        <col />
    </colgroup>
    <!-- Data Column Group -->
    <colgroup>
        <col />
    </colgroup>

    <thead>
        <tr>
            <th>Cons</th>
            <th>Subject</th>
            <th>Pros</th>
        </tr>
    </thead>
    <tbody>
        <tr>
            <td>bad</td>
            <th>Idea</th>
            <td>Good</td>
        </tr>
        <tr>
            <td>No</td>
            <th>Question</th>
            <td>Yes</td>
        </tr>
        <tr>
            <td>Negative</td>
            <th>Pole</th>
            <td>Positive</td>
        </tr>
    </tbody>
</table>