Implement tables and expose styles for addHtml function

This pull request ehances the ```\PhpOffice\PhpWord\Shared\Html::addHtml``` function to :

- Be able to use tables
- Change the default styles
- Use colspan (sorry, I didn't see any reliable way to implement rowspan fonctionnality at the moment, mainly due to the lack of end of node event in the implementation)

Edit: sorry for the 2 failed pull requests before.