Allow for custom handlers in client

[tehmaze]

Not sure how to categorise this, but currently the IMAP client is spewing all kinds of warnings like such:

imap/client: 2016/10/21 17:31:52 response has not been handled: &{* OK HIGHESTMODSEQ [394056] Highest}

Maybe I'm wrong, but I don't seem to be able to specify custom handlers in the client; having such an option would allow me to catch expected "unhandled" messages while preserving the capability to warn if an unexpected unhandled response arises.

My server is running dovecot2 from Debian (jessie) by the way.

[emersion]

Are you sure it's not an issue with https://github.com/emersion/go-imap?