Add support for rowspan in tables

[weaverryan]

After #100, a lot more table structures are supported, but rowspan is still not supported, but could now be added for "pretty/grid" tables: http://docutils.sourceforge.net/docs/ref/rst/restructuredtext.html#grid-tables

[apfelbox]

Sort-of related is that the mapping table of the "types" page is a proper "grid table": https://github.com/doctrine/dbal/blob/master/docs/en/reference/types.rst
But the rendering is broken: https://www.doctrine-project.org/projects/doctrine-dbal/en/2.10/reference/types.html#mapping-matrix

Screenshots

ReST document

In website

[weaverryan]

It's possible that @jwage just hasn't updated that rendering since #100? But indeed, it also may be due to the rowspan issue. Tables are hard - #100 took quite a lot of effort.

[jwage]

The Doctrine website just hasn't been updated to use the latest rst-parser yet.