notmuch.el: add a submap (on "z" for "ztash") to stash things.

Provide key bindings for stuffing various RFC822 header fields and other metadata
into the emacs kill-ring as text. The bindings are as follows:

z F		notmuch-show-stash-filename
z T		notmuch-show-stash-tags
z c		notmuch-show-stash-cc
z d		notmuch-show-stash-date
z f		notmuch-show-stash-from
z m		notmuch-show-stash-message-id
z s		notmuch-show-stash-subject
z t		notmuch-show-stash-to