Add a "null" auth backend

Not built by default, but can be added with `go build -tags nullauth`.
Enabled by running tokidoki with `-auth.url null://`. Very useful for
simpler debugging when you don't want manage test accounts with actual
passwords.

auth/url.go

@@ -18,6 +18,8 @@ func NewFromURL(authURL string) (AuthProvider, error) {
 		return NewIMAP(u.Host, true), nil
 	case "pam":
 		return NewPAM()
+	case "null":
+		return NewNull()
 	default:
 		return nil, fmt.Errorf("no auth provider found for %s:// URL", u.Scheme)
 	}