We should support outlines on circles.

Our "outline"-like styles are inconsistent. On fills we only support 1px outlines. On symbols we support arbitrary width "halos" (which can also be blurred).

Ideally, we would support all of the following properties for circle outlines:

• circle-stroke-color
• circle-stroke-width
• circle-stroke-opacity
• circle-stroke-blur

ref mapbox/mapbox-gl-js#1713

cc @bhousel @vicapow