[emacs] message list view

[David Bremner]

Discussion on #notmuch the other day led me to wonder how useful a
"message list" view would be. Essentially this would display a single
line summary for individual messages matching a query and allow
e.g. tagging operations or "opening" the message to get a more complete
view.

I'm not sure if it's worth the trouble of maintaining another view.  I
think that "notmuch show --entire-thread=false --body=false
--format=sexp" is enough for the CLI backend.

d