Add artboard background color to metadata

[koenbok]

For artboards, I'd love to have the background color included in the metadata:

[
    {
        "id": "F2574D4F-02D2-4627-AC94-9CDFD466821E", 
        "imageType": "png", 
        "kind": "artboard", 
        "layerFrame": {
            "height": 667, 
            "width": 375, 
            "x": 130, 
            "y": 14
        }, 
        "maskFrame": null, 
        "metadata": {
            "backgroundColor": "rgba(255, 0, 0, 1)",
        }, 
        "name": "iPhone6", 
        "visible": true
    }
]