code: function promptingHelper( generator, generator_prompts ) {
    var prompts = filterPrompts( generator.options.PromptAnswers, generator_prompts );
  
    return generator.prompt( prompts )
      .then(
        function ( answers ) {
          addPromptAnswers( generator.options.PromptAnswers, answers );
        }
      );
  }
docs: filters prompts for a generator
  
  compares prompts in the provided generator_prompts with those in generator.options.PromptAnswers.
  if a prompt.name already exists in generator.options.PromptAnswers.answers, then it’s not
  presented again to the user. the generator can retrieve the previously answered prompt with
  generator.options.PromptAnswers.get( prompt.name )
  
  @param {Object} generator
  @param {Array} generator_prompts
  @returns {Promise}