Implement configurable auth providers

New providers need to "register" their URL scheme of choice in
auth.NewFromURL().

Implements: https://todo.sr.ht/~sircmpwn/tokidoki/1

auth/url.go

@@ -0,0 +1,22 @@
+package auth
+
+import (
+	"fmt"
+	"net/url"
+)
+
+func NewFromURL(authURL string) (AuthProvider, error) {
+	u, err := url.Parse(authURL)
+	if err != nil {
+		return nil, fmt.Errorf("error parsing auth URL: %s", err.Error())
+	}
+
+	switch u.Scheme {
+	case "imap":
+		return NewIMAP(u.Host, false), nil
+	case "imaps":
+		return NewIMAP(u.Host, true), nil
+	default:
+		return nil, fmt.Errorf("no auth provider found for %s:// URL", u.Scheme)
+	}
+}