Re-introduce original attach and webLinks addons back

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

src/public/Terminal.test.ts

@@ -0,0 +1,17 @@
+/**
+ * Copyright (c) 2016 The xterm.js authors. All rights reserved.
+ * @license MIT
+ */
+
+import { assert } from 'chai';
+import { Terminal } from './Terminal';
+import * as attach from '../addons/attach/attach';
+
+ describe('Terminal', () => {
+  it('should apply addons with Terminal.applyAddon', () => {
+    Terminal.applyAddon(attach);
+    // Test that addon was applied successfully, adding attach to Terminal's
+    // prototype.
+    assert.equal(typeof (<any>Terminal).prototype.attach, 'function');
+  });
+});