Add multiple viewPortSizes

[ludder]

It would be handy to be able to set multiple viewPortSizes without duplicating the other settings.

Something like:

options: {
        screenshots: 'test/visual',
        results: 'results/visual',
        viewportSize: ["1024, 768", "800, 600", "320, 480"]
      },
      src: [
        'test/visual/**.js'
      ]