RST simple tables: support col spans

```
=====  =====  ======
   Inputs     Output
------------  ------
  A      B    A or B
=====  =====  ======
False  False  False
True   False  True
False  True   True
True   True   True
=====  =====  ======
```
https://github.com/docutils/docutils/blob/master/docutils/docs/ref/rst/restructuredtext.rst#simple-tables
