Add support for shadow DOM.

[shans]

No description provided.

[dstoc]

lgtm

[dfreedm]

One nit here: A node can have N shadowroots stacked on top of each other from "oldest" to "youngest". To copy all the shadows, something like this is required:

function copyShadowRoot(node, sr) {
  if (sr) {
    // recreate shadowroot stack oldest -> youngest
    if (sr.olderShadowRoot) {
      copyShadowRoot(node, sr.olderShadowRoot);
    }
    var nsr = node.createShadowRoot();
    forEach(sr.children, function(c) { nsr.appendChild(sr.cloneNode(true) });
  }
}
copyShadowRoot(copyNode, node.shadowRoot);