Add Emacs' imenu support in notmuch-show and notmuch-search

Emacs' major modes can facilitate navigation in their buffers by
supporting Imenu. In such major modes, launching Imenu (M-x imenu)
makes Emacs display a list of items (e.g., function definitions in a
code buffer). Selecting an item from this list moves point to this
item.

This patch adds Imenu support to both notmuch-show and notmuch-search
buffers:

* in notmuch-show, Imenu will present a list of all messages in the
  currently visible thread;

* in notmuch-search, Imenu will present a list of all messages in the
  search buffer.