feat(config): Add forceJSONP option

This allows to pass this option to socket.io which is needed to support IE7-9.

docs/config/01-configuration-file.md

@@ -275,6 +275,12 @@ will return exit-code `0` and display a warning.
 
 See [config/files] for more information.
 
+## forceJSONP
+**Type:** Boolean
+
+**Default:** `false`
+
+**Description:** Force socket.io to use JSONP polling instead of XHR polling.
 
 ## frameworks
 **Type:** Array

lib/config.js

@@ -253,6 +253,7 @@ var Config = function () {
   this.reportSlowerThan = 0
   this.loggers = [constant.CONSOLE_APPENDER]
   this.transports = ['polling', 'websocket']
+  this.forceJSONP = false
   this.plugins = ['karma-*']
   this.defaultClient = this.client = {
     args: [],

lib/server.js

@@ -30,7 +30,8 @@ function createSocketIoServer (webServer, executor, config) {
     // avoid destroying http upgrades from socket.io to get proxied websockets working
     destroyUpgrade: false,
     path: config.urlRoot + 'socket.io/',
-    transports: config.transports
+    transports: config.transports,
+    forceJSONP: config.forceJSONP
   })
 
   // hack to overcome circular dependency