Added function to check state of notifications

Sometimes is a good idea to check if notifications are suspended,
for example, for changing the icon. This function just returns the value
of suspended.

Closes https://github.com/awesomeWM/awesome/pull/222.

lib/naughty/core.lua.in

@@ -143,6 +143,11 @@ for s = 1, capi.screen.count() do
     }
 end
 
+--- Notification state
+function naughty.is_suspended()
+    return suspended
+end
+
 --- Suspend notifications
 function naughty.suspend()
     suspended = true