Implements the reST flat-table directive.

The ``flat-table`` is a double-stage list similar to the ``list-table`` with
some additional features:

* column-span: with the role ``cspan`` a cell can be extended through
  additional columns

* row-span: with the role ``rspan`` a cell can be extended through
  additional rows

* auto span rightmost cell of a table row over the missing cells on the right
  side of that table-row.  With Option ``:fill-cells:`` this behavior can
  changed from *auto span* to *auto fill*, which automaticly inserts (empty)

list tables

  The *list tables* formats are double stage lists. Compared to the
  ASCII-art they migth be less comfortable for readers of the
  text-files. Their advantage is, that they are easy to create/modify
  and that the diff of a modification is much more meaningfull, because
  it is limited to the modified content.

The initial implementation was taken from the sphkerneldoc project [1]

[1] https://github.com/return42/sphkerneldoc/commits/master/scripts/site-python/linuxdoc/rstFlatTable.pySigned-off-by: Markus Heiser <markus.heiser@darmarIT.de>
[jc: fixed typos and misspellings in the docs]
Signed-off-by: Jonathan Corbet <corbet@lwn.net>