Add a canceller option (a promise) to the Request
Add a canceller option (a promise) to the Request

if the canceller promise is fulfilled before the request is done,
it'll be aborted and the fetch promise rejected. this makes it possible to
implement timeouts to fetch or just canceling any pending requests at will.

this is similar to what the angular $http service does with its `timeout` option,
but without the automatic wrapping of a number in a $timeout promise.

gdamjan/fetch,gdamjan/fetch,gdamjan/fetch,gdamjan/fetch