title: Using quill modules with vue2-editor and webpack mix
body: <p>I'm currently using vue2-editor and importing quill modules and registering them as per documentation.But getting error window.Quill is undefined.</p>
  
  <p>I've tried webpack plugin mix to include window.Quill and Quill but still error remains the same.</p>
  
  <p>In my vue component</p>
  
  <pre><code>import { VueEditor } from "vue2-editor";
  import { Quill } from "quill";
  import { ImageDrop } from "quill-image-drop-module";
  import { ImageResize } from "quill-image-resize-module";
  Quill.register("modules/imageDrop", ImageDrop);
  Quill.register("modules/imageResize", ImageResize);
  </code></pre>
  
  <p>And in my webpack mix</p>
  
  <pre><code>mix.extend('foo',new class{
      webpackPlugins(){
          return new webpack.ProvidePlugin({
              "window.Quill": "quill/dist/quill.js",
              Quill: "quill/dist/quill.js"
      });
      }
  });  
  </code></pre>
  
  <p>Uncaught TypeError: Cannot read property 'imports' of undefined</p>
  
  <p>which is from window.Quill.imports</p>