feat(api): Intent to ship subchart method

[netil]

Issue

#3342

Details

Implement .subchart() and .subchart.reset()

const chart = bb.generate({ ... });

// select subchart selection giving domain range
chart.subchart([2, 5]);

// reset subchart selection
chart.subchart.reset();

[coveralls]

coverage: 91.49% (+0.03%) from 91.456% when pulling a097b50 on netil:subchart#3342 into 21a1ac4 on naver:master.