title: Select element inside dropdown menu issue
body: <p>I am currently experimenting in Element UI a beautiful VueJS frame for ui.
  I am having issue when putting an select element inside a dropdown menu because when I select an item inside the select element it will close the dropdown also.</p>
  
  <p>How should I make the dropdown stay whenever I select an item in select element?</p>
  
  <p>Here is the sample demo. <code>fiddle</code>
  <a href="https://jsfiddle.net/vy70ogbz/" rel="nofollow noreferrer">https://jsfiddle.net/vy70ogbz/</a></p>