Add support for naming a key attribute to use as key values for vnodes

[gryphonmyers]

I needed this feature, so here's a PR.

var myDomNode = document.createElement('div');
myDomNode.id = 'foo';

virtualize(myDomNode, 'id')
/*
will yield something not unlike:
 {
    children: [],
    properties: { id: 'foo' },
    key: 'foo'
  }
*/