Add disconnect method

See https://github.com/Automattic/socket.io-client/issues/737 - "v1.2.0. You can stop reconnection by calling disconnect method now."

socket.io-client/socket.io-client.d.ts

@@ -27,6 +27,7 @@ declare module SocketIOClient {
         emit(event: string, ...args: any[]): Socket;
         listeners(event: string): Function[];
         hasListeners(event: string): boolean;
+        disconnect(): void;
         connected: boolean;
     }