Sibling product swatches: possibility to add custom images as swatches#53

The Sibling product swatches now only support a custom color or the first product image. Adding support for custom images would greatly improve it’s functionality and UI. A product image usually is too small to be used as a swatch and a colored dot might not always best represent the alternative product. Therefor it would be a great improvement if we could use custom images for this, so you can design optimized product swatches for better UI.