feat(treemap): Intent to ship treemap type

[netil]

Issue

#123

Details

Implement Treemap chart type

bb.generate({
  data: {
    columns: [
	["data1", 1300],
	["data2", 200],
        ...
    ],
    type: "treemap", // for ESM specify as: treemap()
    labels: {
      colors: "#fff"
    }
  },
  treemap: {
    label: {
      show: true,
      threshold: 0.03,
      format: function(value, ratio, id) {
	    return value;
       }
    }
  },
});

[coveralls]

Coverage: 90.403% (-0.4%) from 90.77% when pulling 4457ab1 on netil:treemapType#123 into 337c7cb on naver:master.

[stof]

This breaks tree-shaken builds because initClip is not defined at this point. The clip internals are loaded when loading the axis feature. they are not imported in this file.