Add support for Shadow DOM (native or using polyfill)

[taye]

Elements that are in Shadow DOMs can be interacted with.

interact('custom-element')
  .draggable(/* ... */)
  .on('tap', function (event) { /* ... */ });

There are a few issues:

1. Because of what I think might be a bug in Blink, Element.matches always fails with deep/shadow-piercing selectors. For example, try running this on https://polymer-project.org:

   var selector = 'paper-button::shadow *';
document.querySelector(selector).matches(selector); // false

   So such selectors won't work with interact.js.

2. :host, ::shadow, ::content selectors cause errors if used with the Shadow DOM polyfill

3. Dropzones haven't been tested