title: Extract common values from a key and make a multidimensional array
body: <p>I have an <code>array</code> of objects like this:</p>
  
  <pre><code>[
  {"k1":"v1","k2":27172},
  {"k1":"v1","k2":15177},
  {"k1":"v1","k2":19411},
  {"k1":"v2","k2":30163},
  {"k1":"v2","k2":23949},
  {"k1":"v2","k2":23829},
  {"k1":"v2","k2":27821}
  ]
  </code></pre>
  
  <p>What's the easiest way to extract the common values from k1 and create a multidimensional array like the below? </p>
  
  <pre><code>[
  ["v1", [27172, 15177, 19411]],
  ["v2", [30163, 23949, 23829, 27821]]
  ];
  </code></pre>