Add ability to read token reported by `gh auth token`

[ssbarnea]

As official github cli tool has a nice command that exposes its token, this could also be used as a fallback mechanism by ghapi instead of using the less safe environment variable approach.

See https://cli.github.com/manual/gh_auth_token

[jph00]

That sounds like a nice idea-- feel free to do a PR if you'd like to add it (add at-mention me if you do).