Support IMAP4 Login Referrals

I'm wondering if support for [RFC 2221 (IMAP4 Login Referrals)](https://tools.ietf.org/html/rfc2221) is planned?

It's possible to slightly work around this now by checking the error message thrown upon `login` or `authenticate` and parsing the message manually, but it seems like this could be a useful integration.

The current real-world case that prompted this question is a dovecot cluster, where a user's home mailbox exists on another server.
This will result in either:

* `No Response: [REFERRAL imap://test%40mail1.example.com;AUTH=*@mail2.example.com/]` after authentication against the other cluster member.
* Or `No Response: [REFERRAL imap://test%40mail1.example.com;AUTH=*@/]` in the case where the authentication is handled by another cluster member, but the server is unavailable (e.g. due to maintenance).