title: How do you create optgroups in react-select v2?
body: <p>I want to have optgroups in my react-select list, but it doesn't seem to be documented anywhere.  I have the following structure, which I pulled from a comment in <a href="https://github.com/JedWatson/react-select/issues/59" rel="noreferrer">https://github.com/JedWatson/react-select/issues/59</a>:</p>
  
  <pre><code>render() {
    const options = [
      {
        label: "Group 1",
        children: [
          { label: "Group 1, option 1", value: "value_1" },
          { label: "Group 1, option 2", value: "value_2" }
        ]
      },
      { label: "A root option", value: "value_3" },
      { label: "Another root option", value: "value_4" }
    ];
    return &lt;Select options={options} /&gt;
  }
  </code></pre>
  
  <p>I expect "Group 1" to be an optgroup, with options 1 and 2 as children.  Instead, "Group 1" just appears as a regular option.  Does anyone know what the correct key is, within "Group 1", to turn it into an optgroup?</p>
  
  <p>I've already tried "children", and "values", but to no effect.</p>