Add support for experiment after_run blocks

[nickh]

This change allows an experiment to run custom behavior based on experiment results, eg

science("my-experiment") do |e|
  e.use { control_method }
  e.try { candidate_method |

  # Slower candidates need further investigation
  e.after_run do |result|
    control_duration = result.control.duration
    candidate_duration = result.candidates.first.duration

    if candidate_duration - control_duration > 15
      Logger.debug("Slow experiment candidate #{context}")
    end
  end
end