Support HTML5 style alignment with CSS properties instead of just TH align

[jleider]

[michbart]

Hello @domchristie! Would you take a look at this PR, please? I have encountered problem with tables rendered by Redmine RedCarpet Formatter, text alignment is ignored and this change would solve it.

Table rendered by Redmine RedCarpet Formatter:

<table>
  <tbody>
    <tr><th>Name</th><th style="text-align: left;">Surname</th></tr>
    <tr><td>Foo</td><td>Bar</td></tr>
  </tbody>
</table>

Expected output

| Name | Surname |
| --- | :-- |
| Foo | Bar |

Thank you!