Add css_class config to extensions/tables.py

[buaabyl]

Add a config option 'css_class' to tables.py.

[waylan]

Thanks for your submission, however, we are currently in a feature freeze as we work toward version 3.

That said, I still have a few concerns. For starters, the tests were not updated and are failing. Second, the default behavior should remain as it was before this change (do not add a class unless the user specifically provides one). And third, I'm having a hard time seeing the value. As all tables in a document will all receive the same class, you can just select all table elements (from CSS or JavaScript). And if you have other tables in your page template (not from the Markdown document), just wrap the Markdown output in a container and select all child table elements of that container. Without a clear use-case, I'm inclined to reject this.

[buaabyl]

because pygment hightlight using table, and have the same level of other tables, so the wrap not help. I use markdown to write user manual. Tables have different style from highlight table. So I add a custom class to table.