This allows lei to automatically note keyword (message flag)
changes made to a Maildir and propagate it into lei/store:
lei add-watch --state=tag-ro /path/to/Maildir
This doesn't persist across restarts, yet. In the future,
it will be applied automatically to "lei q" output Maildirs
by default (with an option to disable it).
State values of tag-rw, index-<ro|rw>, import-<ro|rw> will all
be supported for Maildir.
This represents a fairly major internal change that's fairly
intrusive, but the whole daemon-oriented design was to
facilitate being able to automatically monitor (and propagate)
Maildir/IMAP flag changes.