Add table generator into Sphinx config extension

Add table generator into Sphinx config extension

New directive .. statementlist:: generates table of statements in a
the given domain (named.conf or rndc.conf). The table contains link to
definition, short description, and also list of tags.
Short description and tags have to be provided by user using optional
parameters. E.g.:

.. statement:: max-cache-size
   :tags: resolver, cache
   :short: Short description

.. statementlist:: is currently not parametrized.

This modification is based on Sphinx "tutorial" extension "TODO".
The main trick is to use placeholder node for .. statementlist:: and
replace it with table at later stage, when all source files were
processed and all cross-references can be resolved.

Beware, some details in Sphinx docs are not up-to-date, it's better
to read Sphinx and docutil sources.