Allow generator-docker to be called with options, and prevent prompt

[dnoliver]

To improve composability, add options to the generator code.

1. use the options as default properties
2. do not prompt for parameters when an option is already defined

i.e:

constructor: function () {
  yeoman.Base.apply(this, arguments);

  this.option('boilerplate', {
      type: Boolean,
      required: false,
      desc: 'Include Boilerplate'
  });
},

initializing: function () {
  this.props = {
      boilerplate: Boolean(this.options.boilerplate)
  };
},

prompting: function () {
  var prompts = [{
      type: 'confirm',
      name: 'boilerplate',
      message: 'Include Boilerplate',
      default: true,,
      when: this.options.boilerplate === undefined
    }];

  return this.prompt(prompts)
      .then(function (props) {
        this.props = extend(this.props, props);
      }.bind(this));
}

[dnoliver]

I can contribute with this feature