d3 pie/donut chart example

Initial version:
https://codepen.io/theacadian/pen/jOPdPGm?editors=1011
Final version:
https://codepen.io/theacadian/pen/ExjregK?editors=0010

References:
- https://schoolofdata.org/2013/10/01/pie-and-donut-charts-in-d3-js/
- http://www.adeveloperdiary.com/d3-js/create-a-simple-donut-chart-using-d3-js/
- https://bost.ocks.org/mike/circles/