voronoi-clipping: now uses multi-value maps for attr

Instead of

 (def svg (-> d3 (.select "body") (.append "svg")
      (.attr "width" width)
      (.attr "height" height)))

We can simply say

(def svg (-> d3 (.select "body") (.append "svg")
      (.attr {:width width :height height})))

Isn't that cool? I think so.

Depends on latest strokes master branch.