notmuch.el: Make notmuch-show buffer name first subject, instead of thread-id (supers...

Change the buffer name to a uniquified subject of the thread (i.e. the
subject of the first message in the thread) instead of the thread-id. This
is more meaningful to the user, and will make it easier to scroll through
numerous open buffers.

Note that this patch adds an optional `buffer-name' argument to notmuch
show.

Signed-off-by: Jesse Rosenthal <jrosenthal@jhu.edu>
Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>