Allow using tab as separator in table

[tsl0922]

source:

right | center
---:    |   :---:
cell1 | cell2
cell3 | cell4

expect:

right	center
cell1	cell2
cell3	cell4

actual:

<table>
  <tbody>
    <tr>
      <td>right</td>
      <td>center</td>
    </tr>
    <tr>
      <td>—:</td>
      <td>:—:</td>
    </tr>
    <tr>
      <td>cell1</td>
      <td>cell2</td>
    </tr>
    <tr>
      <td>cell3</td>
      <td>cell4</td>
    </tr>
  </tbody>
</table>

I'm using the GFM parser, but fix this in the GFM parser requires a lot of change, so the changes are made to the kramdown parser. Since this this a new feature, it may not affect the existing docs.

[gettalong]

Thanks for the pull request!

This is not a bug fix but a feature enhancement and it can potentially impact existing documents. Therefore I would normally postpone this to the 2.0 release of kramdown.

However, this change will probably not affect many existing documents - I will have to think about it a bit more.

[gettalong]

Thanks again for the pull request - this will be in the next release.