refactor(config.js): Extract config file

* Separate different config source to each files
* Freeze config object

lib/realtime.js

@@ -28,7 +28,8 @@ var realtime = {
   secure: secure,
   connection: connection,
   getStatus: getStatus,
-  isReady: isReady
+  isReady: isReady,
+  maintenance: true
 }
 
 function onAuthorizeSuccess (data, accept) {
@@ -699,7 +700,7 @@ function updateHistory (userId, note, time) {
 }
 
 function connection (socket) {
-  if (config.maintenance) return
+  if (realtime.maintenance) return
   parseNoteIdFromSocket(socket, function (err, noteId) {
     if (err) {
       return failConnection(500, err, socket)