Add support for partial arcs to the :circ implementation. Currently :circ only supports full ellipse shapes.

Proposal:

• new property: :angle - specifies the start and end angles of the circle, specified in radians clockwise from x-axis, as per the spec for the similar arguments to CanvasRenderingContext2D.ellipse().
• implementation: modify :circ implementation to check for the presence of :angle property and apply the values to the appropriate arguments of the call to ctx.ellipse(), or else default to a full circle as presently.
• challenges: none expected, should be a fairly easy add.