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/attach.test.ts

@@ -0,0 +1,20 @@
+/**
+ * Copyright (c) 2014 The xterm.js authors. All rights reserved.
+ * @license MIT
+ */
+
+import { assert } from 'chai';
+
+import * as attach from './attach';
+
+class MockTerminal {}
+
+describe('attach addon', () => {
+  describe('apply', () => {
+    it('should do register the `attach` and `detach` methods', () => {
+      attach.apply(<any>MockTerminal);
+      assert.equal(typeof (<any>MockTerminal).prototype.attach, 'function');
+      assert.equal(typeof (<any>MockTerminal).prototype.detach, 'function');
+    });
+  });
+});