don't require maildir_trash. currently MaxMessages gets into the way of
simply removing it; that is fixable with some shuffling, though.

refactor mailbox support. create proper mailbox drivers; handle imap and
maildir (and anything else) symmetrically; decouple UID->message mapping
from sync database - should use the same UID storing schemes as c-client
(pine, uw-imap) does, at least optionally, i think.

add asynchrounous operation to remote mailbox drivers. this is actually
what prevents us from simply using c-client for the previous point and
thus simply becoming mailsync.

store message flags in sync database, so _un_setting them will be properly
synced as well.

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.