title: Extend a dict with anoter dict with cumulative values result
body: <p>I want to extend a given dict with another but if a key is already existing I must accumulate the values.</p>
  <p>Example:</p>
  <p>Let's consider a given function called cumulative_update and two dicts a and b:</p>
  <pre><code>a = { &quot;a&quot; : 1, &quot;b&quot; : 2, &quot;c&quot; : 1 }
  b = { &quot;c&quot; : 3, &quot;d&quot; : 4 }
  a.cumulative_update(b)
  </code></pre>
  <p>The result wanted should be:</p>
  <blockquote>
  <p>{'a': 1, 'b': 2, 'c': 4, 'd': 4}</p>
  </blockquote>
  <p>But, when I use the default append function:</p>
  <pre><code>a.update(b)
  </code></pre>
  <p>I get:</p>
  <blockquote>
  <p>{'a': 1, 'b': 2, 'c': 3, 'd': 4}</p>
  </blockquote>
  <p>I found also a solution to append values in <a href="https://stackoverflow.com/questions/21518271/how-to-sum-values-of-the-same-key-in-a-dictionary">How to sum values of the same key in a dictionary?</a> but it is based on keys which isn't possible in my case.</p>