Framework for help export from wrapper binary.

Framework for help export from wrapper binary.

- Add a separate CommandLineHelpContext.  This class layers extra
  information on top of a HelpWriterContext, specific for command-line
  help export.
- Add a global instance of the above to be able to pass it into
  write_man() through functions unaware of its existence.  Make
  write_man() use the instance if present.
- Add -export option for 'gmx help', and an interface that needs to be
  implemented to export a particular format.

Related to #685 and #969.

Change-Id: Ica16895f8136a09bc5995812c4da5363d097c2b1