Suggestion: support external authenticators

[gkgoat1]

This would help with JS (#61 ) and external OS bindings

[Progdrasil]

This was indeed always in my plans, which is why the authenticator and client are archetected separately. I just never got around to implementing any of it since it wasn't needed internally. We accept PRs should anyone wish to attempt their hand at implementing this.

Thanks for writing it down.