Implement UTF8=ACCEPT capability

[emersion]

See https://tools.ietf.org/html/rfc6855

[foxcpp]

What needs to be done for it? From my understanding of linked RFC and go-imap internals, it needs to be supported by go-imap itself.
But since UTF8 extension relies on ENABLE to activate it, we might need to merge go-imap-enable into go-imap[1] for later to be able to properly handle ENABLE UTF8=ACCEPT.

I'm willing to work on it, but I may need some guidance.

[1] I actually think it is a very good idea to bring all these small extensions into go-imap itself, probably even enabling them by default. IMAP4 without extensions is simply inadequate anyway.

[foxcpp]

Depends on #342 to get ENABLE extension in go-imap itself.