feat(arc): Intent to ship arc.rangeText

[netil]

Issue

#2120

Details

Implement new arc.rangeText option, which can display a range in Arc shape.

arc: {
     rangeText: {
      	values: [15, 30, 50, 75, 95],
      	unit: "%", (default: 'absolute')
      	fixed: false, // only available for gauge chart
      	format: function(v) {
      	  return v === 15 ? "Fifteen" : v;
      	},

      	position: function(v) {
      	  return v === 15 ? {x: 20, y: 10} : null; // can return one props value also.
      	},
      	position: {x: 10, y: 15},
      	position: {x: 10}
     }
}

Gauge	Donut

[coveralls]

coverage: 94.442% (+0.03%) from 94.413%
when pulling be4aeba on netil:arcRangeText#2120
into b8e6aa7 on naver:master.