Export SOCKET_CONFIG_TOKEN (rodgc#188)

This is needed if one needs to extend a basic configuration with
namespaces, for example.

index.ts

@@ -1,3 +1,3 @@
-export { SocketIoModule }    from './src/socket-io.module';
+export { SocketIoModule, SOCKET_CONFIG_TOKEN } from './src/socket-io.module';
 export { SocketIoConfig } from './src/config/socket-io.config';
 export { WrappedSocket as Socket }   from './src/socket-io.service';