code: function addNode (node, nest) {
      var obj = getHeadingObject(node)
      var level = getHeadingLevel(node)
      var array = nest
      var lastItem = getLastItem(array)
      var lastItemLevel = lastItem
        ? lastItem.headingLevel
        : 0
      var counter = level - lastItemLevel
  
      while (counter > 0) {
        lastItem = getLastItem(array)
        if (lastItem && lastItem.children !== undefined) {
          array = lastItem.children
        }
        counter--
      }
  
      if (level >= options.collapseDepth) {
        obj.isCollapsed = true
      }
  
      array.push(obj)
      return array
    }
docs: Add a node to the nested array.
  @param {Object} node
  @param {Array} nest
  @return {Array}