emacs: Add compatibility for org-msg.el

Enable `notmuch-company` with `org-msg`, which does not derive from
`message-mode` in composing. This change was first proposed in the
discussion here:
https://github.com/jeremy-compostella/org-msg/issues/53

NEWS

@@ -8,6 +8,8 @@ Emacs
 previous behaviour of using notmuch to send mail by default, customize
 `mail-user-agent` to `notmuch-user-agent`.
 
+`notmuch-company` now works in `org-msg`.
+
 Vim
 ---
 

emacs/notmuch-company.el

@@ -70,7 +70,8 @@
 	(completion-ignore-case t))
     (cl-case command
       (interactive (company-begin-backend 'notmuch-company))
-      (prefix (and (derived-mode-p 'message-mode)
+      (prefix (and (or (derived-mode-p 'message-mode)
+		       (derived-mode-p 'org-msg-edit-mode))
 		   (looking-back
 		    (concat notmuch-address-completion-headers-regexp ".*")
 		    (line-beginning-position))