handle colspan on table headers

[isaacwhite]

This change fixes an issue where the table formatter currently ignores colspan attributes on th tags. For example:

with this markup:

<table class="capitalization" summary="Capitalization Examples">
  <caption> Headline Capitalization </caption>
  <colgroup>
    <col style="width:15%">
      <col style="width:15%">
  </colgroup>
  <colgroup>
    <col style="width:10%">
      <col style="width:30%">
        <col style="width:30%">
  </colgroup>
  <thead>
    <tr style="background-color:#ddd">
      <th class="tableheader" colspan="2">Capitalize</th>
      <th class="tableheader" colspan="3">lowercase</th>
    </tr>
  </thead>
  <tbody>
    <tr>
      <td> <i>No <br>    Nor <br>    Not</i> </td>
      <td> <i>Off <br>    Out <br>    So <br>    Up</i> </td>
      <td> <i>a <br>    and <br>    as <br>    at <br>    but <br>    by</i> </td>
      <td> <i>en</i> (in <i>en Route</i>)
        <br><i>for <br>    if <br>    in <br>    of <br>    on</i> </td>
      <td> <i>or</i>
        <br><i>the</i>
        <br><i>to</i>
        <br><i>v.</i> (in legal contexts)
        <br><i>vs. <br>    via</i> </td>
    </tr>
  </tbody>
</table>

Results in this text:

CAPITALIZE   LOWERCASE   
No           Off         a      en (in en Route)     or                        
Nor          Out         and    for                  the                       
Not          So          as     if                   to                        
             Up          at     in                   v. (in legal contexts)    
                         but    of                   vs.                       
                         by     on                   via

When really what should be output is this text:

CAPITALIZE           LOWERCASE                                                  
No           Off     a           en (in en Route)     or                        
Nor          Out     and         for                  the                       
Not          So      as          if                   to                        
             Up      at          in                   v. (in legal contexts)    
                     but         of                   vs.                       
                     by          on                   via

This change does not attempt to fix the absence of the <caption> tag's content from the output text, which is also demonstrated by the above markup.

[mlegenhausen]

@isaacwhite Could you please rebase and add a test case?

[KillyMXI]

Version 6 output:

CAPITALIZE   LOWERCASE
No    Off    a     en (in en Route)   or
Nor   Out    and   for                the
Not   So     as    if                 to
      Up     at    in                 v. (in legal contexts)
             but   of                 vs.
             by    on                 via

Cells with colspan and rowspan can make use of extra space across columns/rows now.

Your effort is appreciated, but this is no longer needed.

I made a note about caption and colgroup. Maybe I'll take a look at them later, along with extra formatting for thead, tfoot.