feat(options): Intent to ship bar.overlap

[netil]

Issue

#2839

Details

Implement overlapped bar option

bb.generate({
	data: {
		columns: [
			["data1", 80, 150, 100],
			["data2", 100, 120, 130],
			["data3", 150, 80, 120]
		],
		type: "bar"
	},
	bar: {
		width: {
			data1: 60,
			data2: 40,
			data3: 20
		},
		overlap: true
	}
});

[coveralls]

Coverage decreased (-0.003%) to 90.52% when pulling 048fdef on netil:barOverlap#2839 into ab4d98d on naver:master.