Resolves#54.

Alternative solution to#68.
My implementation differs in the following ways:

• fetch does not override signal's abort handler (one controller can cancel many actions)
• abort results in rejection with DOMExpection and proper error message.

I polyfilled AbortController and AbortSignal in tests, because they do not exist in node env.

To consider:

• will addEventListener prevent GC from clearing unused fetch scopes?

Build:

Build "unfetch" to dist:        556 B: unfetch.js.gz        461 B: unfetch.js.br        558 B: unfetch.mjs.gz        464 B: unfetch.mjs.br        629 B: unfetch.umd.js.gz        525 B: unfetch.umd.js.brBuild "unfetch" to polyfill:        559 B: index.js.gz        466 B: index.js.br