Finished off hooke.hooke and fleshed out hooke.ui.

Finished off hooke.hooke and fleshed out hooke.ui.

Highlights:
  * New hooke.command for CommandExit (and subclasses), Command, and
    Argument.  For the same reason as hooke.playlist: keep core
    functionality out of hooke.plugin, which is only for the pluggable
    interface to that functionality.
  * New hooke.engine containing CommandEngine.  Not much going on
    here.  Perhaps I will merge it into hooke.command later...
  * New hooke.ui.commandline stub for 'command line' UI.
  * Filled in hooke.ui stub.
  * Added UI and CommandEngine handling to hooke.hooke.Hooke.
  * Added a new super-local '.hooke.cfg' to hooke.config.DEFAULT_PATHS.
    For project-specific hooke configuration?  Mostly to avoid suprising
    the user by clobbering ~/.hooke.cfg.
  * Fix expanduser bug in hooke.config.HookeConfigParser.write.
  * Fix super(AddCommand, self) -> super(AddGlobCommand, self) bug in
    hooke.plugin.playlist.AddGlobCommand.__init__.