* Support OAuth2 login flow from CLI (resolves#172)
* Refactor SessionManager to handle local and OAuth2 logins.
* argo login will request permanent credentials after OAuth2 flow
* Implement proper OIDC app state nonce. Add explicit `--sso` flag to `argo login`
* Don't ask for user credentials if username and password are specified as arguments
* Add cli image make target
* Don't re-prompt username/password in PromptCredentials