This provides initial support for postponing in the emacs frontend;
resuming will follow in a later commit. On saving/postponing it uses
notmuch insert to put the message in the notmuch database
Current bindings are C-x C-s to save a draft, C-c C-p to postpone a
draft (save and exit compose buffer).
Previous drafts get tagged deleted on subsequent saves, or on the
message being sent.
Each draft gets its own message-id, and we use the namespace
draft-.... for draft message ids (so, at least for most people, drafts
are easily distinguisable).
