title: group array of object by dates
body: <p>I have a data structure that look like this</p>
  
  <pre><code>const arr = [{
      name: "Alice",
      created_at : "2017-04-18"
  },
  {
      name: "James",
      created_at : "2017-06-30"
  },
  {
      name: "Melisa",
      created_at : "2017-04-03"
  },
  {
      name: "Amy",
      created_at : "2017-05-03"
  }];
  </code></pre>
  
  <p>I want to restructure it so that I can display them. I want to display them by month, like so</p>
  
  <p>April 
  - Alice
  - Melisa</p>
  
  <p>May
  - Amy</p>
  
  <p>June
  - James</p>
  
  <p>I have no clue where to begin.</p>