add enable() method to Typeahead

Summary: adds an enable() method to Typeahead.  Use case is the following: I have a tokenizer and when a certain class of tokens is selected I want to disable the typeahead.  When those tokens are removed I want to re-enable the typeahead.

Test Plan: Implemented tokenizer with this behavior

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D3617

src/lib/control/typeahead/Typeahead.js

@@ -322,6 +322,15 @@ JX.install('Typeahead', {
     },
 
 
+    /**
+     * @task control
+     */
+    enable : function() {
+      this._control.disabled = false;
+      this._stop = false;
+    },
+
+
     /**
      * @task control
      */