title: lodash - Group/sum two arrays into object
body: <p>I have two arrays (one of keys and one of values) that I want to group into an object with summing the values when keys are identical.</p>
  
  <pre><code>var keys = ['a', 'b', 'a', 'c'];
  var values = [1, 2, 2, 3];
  </code></pre>
  
  <p>I have tryed using lodash <code>zibObject()</code> but there is not way to sum the value with this function. I guess using <code>zipWith()</code> would be the solution, but I dont know how to sum.</p>
  
  <pre><code>var grouped = _.zipWith(keys, values, function(a,b){
      return {a: b}; // not summing: [{a: 1}, {b: 2}, {a, 2}, {c: 3}]
  });
  </code></pre>
  
  <p>but what I want is:</p>
  
  <pre><code>var result = {
      a: 3,
      b: 2,
      c: 3
  };
  </code></pre>
  
  <p>What would be the proper way to achieve that with lodash?</p>