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 ignores asynchronous notifications (untagged responses), so mail 
arriving during a fetch will not be fetched in the current run any more.

add a way to automatically create and sync IMAP subfolders.

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