code: function checkboxNode(selection) {
    selection.append('span').classed('arrow', true);
    selection.append('input')
        .attr('type', 'checkbox');
    selection.append('span')
        .classed('label', true);
  }
docs: Checkbox node (A basic implementation of nodeEnterFactory)
  @param {d3.selection} selection - selection of node content (li element)