Added the newmail_hook

When new mail arrives, this hook is triggered, allowing the user to
play a sound, or launch a popup.

Signed-off-by: Matthew Krafczyk <krafczyk.matthew@gmail.com>

offlineimap.conf

@@ -295,6 +295,19 @@ remoterepository = RemoteExample
 #postsynchook = notifysync.sh
 
 
+# This option stands in the [Account Test] section.
+#
+# You can specify a newmail hook to execute an external command upon receipt
+# of new mail in the INBOX.
+#
+# This example plays a sound file of your chosing when new mail arrives.
+#
+# This feature is experimental.
+#
+#newmail_hook = lambda: os.sytem("cvlc --play-and-exit /path/to/sound/file.mp3" +
+#				 " > /dev/null 2>&1")
+
+
 # This option stands in the [Account Test] section.
 #
 # OfflineImap caches the state of the synchronisation to e.g. be able to