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.

Please keep line length under 80 characters when it is easy to do so. And in any case don't go over 120.
120 is the limit displayed by GitHub on desktops. 80 is the limit displayed by GitHub on mobiles.

In this case, I think one line per parameter (here, and in the construction of the literal) is a good idea, as it improves readability. After all, in JavaScript, you would use one line per pair `key: value`.