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.

emacs/notmuch-address.el

@@ -98,6 +98,17 @@ to know how address selection is made by default."
   :group 'notmuch-send
   :group 'notmuch-external)
 
+(defcustom notmuch-address-completion-hook nil
+  "Functions called after completing address.
+
+The completed address is passed as an argument to each function.
+Note that this hook will be invoked for completion in headers
+matching `notmuch-address-completion-headers-regexp'.
+"
+  :type 'hook
+  :group 'notmuch-address
+  :group 'notmuch-hooks)
+
 (defun notmuch-address-selection-function (prompt collection initial-input)
   "Call (`completing-read'
       PROMPT COLLECTION nil nil INITIAL-INPUT 'notmuch-address-history)"
@@ -206,7 +217,8 @@ external commands."
 	  (progn
 	    (push chosen notmuch-address-history)
 	    (delete-region beg end)
-	    (insert chosen))
+	    (insert chosen)
+	    (run-hook-with-args 'notmuch-address-completion-hook chosen))
 	(message "No matches.")
 	(ding))))
    (t nil)))

emacs/notmuch-company.el

@@ -86,6 +86,7 @@
       (match (if (string-match notmuch-company-last-prefix arg)
 		 (match-end 0)
 	       0))
+      (post-completion (run-hook-with-args 'notmuch-address-completion-hook arg))
       (no-cache t))))