code: function insert (parent, children) {
    if (!parent || !children) {
      return
    }
    let nodes = Array.prototype.slice.call(children)
    if (nodes.length === 0) {
      nodes.push(children)
    }
    for (let i = 0; i < nodes.length; i++) {
      if (this.contains(nodes[i], parent)) {
        continue
      }
      if (nodes[i] !== parent && parent.appendChild) {
        parent.appendChild(nodes[i])
      }
    }
  }
docs: Insert dom list to a node
  
  @param  {HTMLElement} parent the node will be inserted
  @param {Array} children node list which will insert into parent