v2: client status

[emersion]

Continuation of https://github.com/emersion/go-imap/pull/490

• [ ] Consider adding separate types for FetchItemData{Body,Binary}Section.Section
• [x] Add support for LITERAL+
• [x] ~~Add IMAP4rev1 fallback for IDLE~~ (#543)
• [x] Consider restarting IDLE every 30min (#542)
• [x] Consider setting SEARCH charset for IMAP4rev1
• [ ] ENABLE IMAP4rev2 on servers which also support IMAP4rev1
• [x] ~~Consider turning SearchCriteria.Not into a pointer, instead of a slice~~ (#505)
• [x] Add support for NAMESPACE
• [x] Add support for ESEARCH
• [x] Add support for SEARCHRES
• [x] Decode literal8
• [ ] Make command methods extensible (see RFC 4466)
• [x] Canonicalize message flags and mailbox attrs
• [x] Parse date in envelope
• [x] Add tests which can run against any IMAP server
• [x] Consider turning lists of items into bool fields (e.g. SearchOptions.Return)
• [x] Drop Client.StartTLS in favor of a function which constructs an already-secured TLS connection
• [ ] Ignore responses that don't make sense given the connection state (section 11.3)
• [ ] Consider re-using the same *FetchBodySection pointers in the returned map, or make it easier to match a *FetchBodySection from a command
• [ ] Drop Command because it's not extensible
• [ ] Some values can be either seq num or UID, how to represent these? Examples: SearchData.Min/Max, SortCommand.Wait, ThreadData.Chain. Should we add separate UID types for each?
• [x] Add support for more extensions
  • [x] METADATA
  • [x] QUOTA (#492)
  • [x] SORT
  • [x] THREAD