notmuch.el: Add 'N' binding to mark message read and go to next.

The magic space bar is nice, but sometimes there's a message with a
long attachment that I just want to skip, but still consider the
message marked as read.

TODO

@@ -1,5 +1,9 @@
 Emacs interface (notmuch.el)
 ----------------------------
+Make hidden components self documenting, such as:
+
+	[24-line citation, press 'c' to display.]
+
 Add support to compose a reply to the current messaage.
 
 notmuch command-line tool

notmuch.el

@@ -33,6 +33,7 @@
     (define-key map "c" 'notmuch-show-toggle-citations-visible)
     (define-key map "h" 'notmuch-show-toggle-headers-visible)
     (define-key map "n" 'notmuch-show-next-message)
+    (define-key map "N" 'notmuch-show-mark-read-then-next-open-message)
     (define-key map "p" 'notmuch-show-previous-message)
     (define-key map (kbd "C-n") 'notmuch-show-next-line)
     (define-key map (kbd "C-p") 'notmuch-show-previous-line)