Add AbortController typings

[coldhamix]

Abortable fetch is landed whatwg/fetch/pull/523, so AbortController, AbortSignal and fetch property in RequestInit should be added to the typescript library

[mhegazy]

PRs welcomed. You can find more information about contributing lib.d.ts fixes at https://github.com/Microsoft/TypeScript/blob/master/CONTRIBUTING.md#contributing-libdts-fixes.