title: tree-model-js how to get previous node id
body: <p>I wanted to know the previous visited node from tree. 
  trying with below example </p>
  
  <pre><code>var TreeModel = require('tree-model');
  tree = new TreeModel();
  
  rootMain = tree.parse({
      id: 1,
      children: [
          {
              id: "11",
              children: [{id: "111"}]
          },
          {
              id: "12",
              children: [{id: "121"}, {id: "122"}]
          },
          {
              id: "13"
          }
      ]
  });
  </code></pre>
  
  <p>If suppose I traverse to the node 121 and 122 I wanted the parent node then it should return me the 12 
  If suppose I traverse to the node 111 I wanted the parent node then it should return me the 11
  If suppose I traverse to the node 13 I wanted the parent node then it should return me the 1</p>