Provide default autoprogram's prog option

[dirn]

When processing subcommands, sphinxcontrib-autoprogram seems to pick up sphinxbuild as the program. Fortunately it provides a way to override this: the prog option. Anyone documenting their command line extension will most likely use something like the following:

.. hensoncli:: path.to:MyExtension
   :prog: henson --app APP_PATH
   :start_command: myext

HensonCLIDirective should provide henson --app APP_PATH as the default value so that extension developers don't need specify it themselves.