[heycam/webidl] Getting rid of optional (#905)

If we do #60, should we get rid of optional and instead use defaulting as sole mechanism?
```
  [NewObject] Document createHTMLDocument(optional DOMString title);
```
would become
```
  [NewObject] Document createHTMLDocument(DOMString title = void);
```
and
```
  void observe(Node target, optional MutationObserverInit options = {});
```
would become
```
  void observe(Node target, MutationObserverInit options = {});
```
(And if you need to distinguish between not given and undefined you use overloading, as XMLHttpRequest does for `open()`.)

cc @saschanaz @tabatkins 

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