Extended the label config to support styles by key

[miekademarco]

the changes will allow you to customise the style of labels for every item.

eg:

 labels: {
    markers: {
        render: function (index) {
            return markers[index].labelText;
        },
        offsets: function (index) {
            return markers[index].offset;
        },
        styles: function (index) {
            return { fill: markers[index].labelFill };
        }
    }
}