emacs: add notmuch-address-post-completion-hook

emacs: add notmuch-address-post-completion-hook

This hook can be used to update the message based on the results of
address completion. For example using message-templ or gnus-alias to set
the From address based on the To address just completed.

The post-completion command is added to the notmuch-company backend to
ensure that the hook is also called company completion is started
without going through notmuch-address-expand-name. See the docstring of
`company-backends' for more information.