User Guide: Generate command line options table from ghc-flags directives

Since the move to ReStructuredText and Sphinx we've generated the options summary tables and manpage with utils/mkUserGuidePart. The options are described in a set of Haskell modules which are a pain to edit and may easily fall out of date.

It would be far better if we could generate these tables from the .. ghc-flag directives already present in the users guide. This will involve a bit of hacking with Sphinx, although it should be more than up to the task.