selected rounding of corners in d3.arc()

[wyhleonard]

I add two attributes of d3.arc(): selectedCorner_s and selectedCorner_e, which control the rounding process of corners at start-angle or end-angle. The default value of these attributes is 1. Users can close the rounding process of corners at start-angle(end-angle) by setting the value of selectedCorner_s(selectedCorner_e) as 0.