Single Sign On

I'm not sure if this has been brought up before, or if this is even within scope, but it would be great if keybase were able to make it possible to login to websites using my GPG key. I currently use `pass` to encrypt my (randomly generated) passwords using GPG, but it would make more sense to cut out the password middle-man entirely. For example:
1. go to a website and click "login with keybase"
2. the website connects to keybase (perhaps with something like OpenID or OAuth)
3. keybase forwards the request to either the CLI client or the in-browser client
4. I sign/decrypt an appropriate challenge with my GPG key to verify my identity
5. an appropriate response is sent back to the original website to authenticate

Ideally this could be done without even needing either party to trust keybase.io, but don't ask me how :)

The issue describes a proposed website login flow using Keybase and a GPG key, but names no files, tests, or implementation entry points. Start by determining whether Keybase has an existing authentication, OpenID, OAuth, CLI, or browser-client integration; done would require an agreed design and an implemented, tested end-to-end flow.

Written by the indexing model from the issue text.