Re-introduce original attach and webLinks addons back

We could release this in v3 if we don't break addons like this

src/addons/attach/Interfaces.ts

@@ -0,0 +1,23 @@
+/**
+ * Copyright (c) 2018 The xterm.js authors. All rights reserved.
+ * @license MIT
+ *
+ * Implements the attach method, that attaches the terminal to a WebSocket stream.
+ */
+
+import { Terminal, IDisposable } from 'xterm';
+
+export interface IAttachAddonTerminal extends Terminal {
+  _core: {
+    register<T extends IDisposable>(d: T): void;
+  };
+
+  __socket?: WebSocket;
+  __attachSocketBuffer?: string;
+  __dataListener?: IDisposable;
+
+  __getMessage?(ev: MessageEvent): void;
+  __flushBuffer?(): void;
+  __pushToBuffer?(data: string): void;
+  __sendData?(data: string): void;
+}