Support timeout on fetch requests

[ramboz]

Is your feature request related to a problem? Please describe.
Running multiple fetch requests in parallel may end up causing denial-of-service attacks if they are long-lived.

Describe the solution you'd like
Regular XHR requests support a timeout that will fail the request after a specific amount of time.
There is currently no clear spec for it in the fetch API, but some discussion around it exist (see links below)

Would be great to have a first draft implementation in the library to offer a decent enough implementation until the spec is finalized.

Describe alternatives you've considered
This can alternatively be implemented consumer-side by using timeouts and promises, and rejecting the promise before the request ends, but the socket is still left open until the fetch actually finishes so it doesn't fully removes the issue

Additional context
See:

• Add timeout option whatwg/fetch#20
• Add option to reject the fetch promise automatically after a certain time elapsed (with no API for arbitrary aborts) whatwg/fetch#179
• Add option to automatically abort response stream if the data stopped arriving for longer than a specified time whatwg/fetch#180

[stefan-guggisberg]

supported in v1.1.0