Support Border Radius for Donut, Pie and Polar Area Charts

[Thimows]

Summary

A very nice feature would be the ability to set the border radius of the Donut, Pie and Polar Area charts. This would allow us to make charts that look like this:

API Changes

This setting could live in the plotOptions for the Donut, Pie and Polar Area charts and can be called borderRadius (just like for the bar charts)