Feature request: Opt-out of activeFill highlighting

[jsjohann]

Thanks for sharing this great library! We use reagraph to show the relations between different nodes. Each node is assigned a specific color which gets overwritten when highlighted (e.g. active or selected) at the moment. It would be great to opt-out of this behaviour via a new property or something like that, so that the color stays the same. The additional ring when selected or the adjusted opacity would be enough for us to differentiate between the states (highlighted or not highlighted).