Implment insertAdjacentHTML as a JSDOM patch

Currently insertAdjacentHTML (
    https://developer.mozilla.org/en-US/docs/Web/API/element.insertAdjacentHTML
    )

JSDOM doesn't currently implment the insertAdjacentHTML DOM element
method.  I've added an implementation based on this polyfill:
https://gist.github.com/eligrey/1276030