strokes: simple venn diagram

A minimalist example of a strokes program: 3 circles. This example highlights the succinctness of clojure maps when setting attributes. In addition to d3's selection.attr maps can be used on selection.style, selection.classed, selection.property, selection.on, transition.attr and transition.style.

Inspired by Mike's #1067616, but with using d3 multi-value maps for attributes as in #3305515 and with the styling moved into the css.