Support gradient legends

Legends can be based on icons or color squares right now; there's a desire to support a true gradient.

This would involve allowing the user to provide a list of colors to create a gradient in addition to providing labels (I will aspire to allowing more than just 2 labels but at the minimum one for the top and one for the bottom)

Here's an example of a gradient (though SimpleMaps is leaflet based so the gradient will appear as a label on top of the map rather than to the side).