mu4e: Place provide forms in their own sections

Use "_" as the title of that section so that it is less distracting
when sections are collapsed to get an overview of the library.

Using a separate section is useful because it reduces the risk of
accidentally into the middle of a library.

mu4e/mu4e-contrib.el

@@ -216,7 +216,7 @@ buffers found, compose a new message and then attach the file."
                    (setq files-to-attach (cdr files-to-attach)))
                  (message "Attached file(s) %s" files-str))
         (message "No buffer to attach file to.")))))
-;;; end of eshell functions
 
+;;; _
 (provide 'mu4e-contrib)
 ;;; mu4e-contrib.el ends here