Added new label rendering mode "aside" to render labels in two vertical stacks instead of around the circle.

[mnori]

I described an issue in #160 where clusters of small segments at the bottom of the pie cause label lines to overlap:

I couldn't work out how to fix this and keep the circular label positioning, but did end up creating a new alternative label positioning mode called "aside" where the labels are rendered as two vertically aligned lists, this gets rid of the problem:

The new rendering mode can be activated with a config option:

{	
	...
	"labels": {
		"lines": {
			"enabled": true,
			"color": "#777777",
			"style": "aside" // <- new style option
		}
	}
	...
}

Demo of the bug and the new "aside" rendering mode can be found here: https://codepen.io/wildtype/pen/ajjrmr