title: Webpack warning when building d3-tip
body: <p>I've included <code>d3-tip</code> in my <code>d3</code> implementation by using the following code:</p>
  
  <pre><code>import * as d3 from "d3";
  import * as d3Tip from "d3-tip";
  d3.tip = d3Tip;
  </code></pre>
  
  <p>However, I'm getting the following warning:</p>
  
  <pre><code>WARNING in ./~/babel-loader/lib!./~/vue-loader/lib/selector.js?type=script&amp;index=0!./src/components/opsboard/opsboard.vue
  75:16-22 "export 'tip' (imported as 'd3') was not found in 'd3'
  </code></pre>
  
  <p>Anyone got any ideas what's wrong?  The tip is working correctly.  Because I'm assigning <code>d3.tip</code> to be <code>d3Tip</code> I don't get why it's exporting it...</p>