[dom] Pull Request: Add EventListenerOptions

RByers has just submitted a new pull request for 
https://github.com/whatwg/dom:

== Add EventListenerOptions ==
My first crack at incorporating the ideas and discussion from my 
[EventListenerOptions 
proposal](https://github.com/RByers/EventListenerOptions) into the 
spec.  The result can be previewed 
[here](https://rawgit.com/RByers/dom/event-listener-options/dom.html).

One thing I'm still not super happy with is the somewhat confusing 
defaults for `mayCancel`.  On the one hand we 
[agreed](https://github.com/RByers/EventListenerOptions/issues/17) 
that we wanted the performance behavior (`mayCancel:false`) the be the
 default when using the new API form.  But we of course can't [change 
the existing 
behavior](https://github.com/RByers/EventListenerOptions/issues/22).  
Let's discuss that further 
[here](https://github.com/RByers/EventListenerOptions/issues/17) if 
desired.


See https://github.com/whatwg/dom/pull/82