Handle the AbortSignal in get() and create() themselves. (#110)

* added aborted flag and gitignore file

* minor editorial tweak

* fixed minor issues

* added abort signal to request and create options

* updated src

* minor grammatical fixes

* fix up minor details

* Handle the AbortSignal in get() and create() themselves.

And remove WebAuthn-specific text.

Thanks to AngeloKai for the basis of this change.

* Describe the internal methods' use of AbortSignal in Extension Points

Instead of by the definition of the internal methods.

* Make the WebAuthn reference non-normative.

Author: jyasskin