Add MutationObserver

MDN: https://developer.mozilla.org/en-US/docs/Web/API/MutationObserver
W3C: http://www.w3.org/TR/dom/#interface-mutationobserver

Looks like MutationObserver is an experimental API ([from the list](https://developer.mozilla.org/en-US/docs/Web/API)), but it seem to be supported by all modern browsers, and other experimental APIs like [AnimationEvent](https://github.com/scala-js/scala-js-dom/blob/master/src/main/scala/org/scalajs/dom/lib.scala#L6281), [DOMParser](https://github.com/scala-js/scala-js-dom/blob/master/src/main/scala/org/scalajs/dom/lib.scala#L5278) are already in scala-js-dom.

That's the idea. Now if there are optional elements, they should probably be typed as `js.UndefOr`, and the parameters of `apply` have a default value of `js.undefined`.