title: Merge all list values together in a map
body: <p>I would like to convert a Map like:</p>
  
  <pre><code>Map&lt;String, List&lt;String&gt;&gt; 
  </code></pre>
  
  <p>to </p>
  
  <pre><code>List&lt;String&gt; 
  </code></pre>
  
  <p>where the result list is the merge of all List values. </p>