don't require maildir_trash

refactor mailbox support.
put sync info outside the mailbox itself.

store flags in sync info.

add option to suppress upload of messages (but not deletion).

add support for syncing with other: and shared: via NAMESPACE

isync gets confused when new mail is delivered while in the middle of an
IMAP session.  need to handled those asynchronous notifications properly.

add a way to automatically create and sync IMAP subfolders.

possibly use ^[[1m to highlight error messages.