Use this to register a customer output formatter.
# File lib/spec/story/runner.rb, line 36
36: def register_listener(listener)
37: story_runner.add_listener(listener) # run_started, story_started, story_ended, #run_ended
38: world_creator.add_listener(listener) # found_scenario, step_succeeded, step_failed, step_failed
39: scenario_runner.add_listener(listener) # scenario_started, scenario_succeeded, scenario_pending, scenario_failed
40: end