title: Vulcanize produces repeated scripts
body: <p>I have created two Polymer custom elements. Both of them uses moment.js. So I have referenced it in both the elements, like:
  <code>&lt;script src="../../bower_components/moment/min/moment.min.js"&gt;&lt;/script&gt;
  </code>
  I use vulcanize to produce a single import file element.html. The issue is that the content of moment.js appears twice in the vulcanized file. 
  Is this correct? Is there a way to avoid it?</p>