v2: client status

[emersion]

Continuation of #490

• Consider adding separate types for FetchItemData{Body,Binary}Section.Section
• Add support for LITERAL+
• Add IMAP4rev1 fallback for IDLE (imapclient: add IDLE fallback #543)
• Consider restarting IDLE every 30min (imapclient: restart IDLE automatically #542)
• Consider setting SEARCH charset for IMAP4rev1
• ENABLE IMAP4rev2 on servers which also support IMAP4rev1
• Consider turning SearchCriteria.Not into a pointer, instead of a slice (Make SearchCriteria.Not a pointer #505)
• Add support for NAMESPACE
• Add support for ESEARCH
• Add support for SEARCHRES
• Decode literal8
• Make command methods extensible (see RFC 4466)
• Canonicalize message flags and mailbox attrs
• Parse date in envelope
• Add tests which can run against any IMAP server
• Consider turning lists of items into bool fields (e.g. SearchOptions.Return)
• 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
• 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?
• Add support for more extensions
  • METADATA
  • QUOTA ([v2] imapclient: add support for QUOTA #492)
  • SORT
  • THREAD