Add chainable definitions for _(...).keys

lodash/lodash-tests.ts

@@ -909,6 +909,7 @@ result = <boolean>_.isString('moe');
 result = <boolean>_.isUndefined(void 0);
 
 result = <string[]>_.keys({ 'one': 1, 'two': 2, 'three': 3 });
+result = <string[]>_({ 'one': 1, 'two': 2, 'three': 3 }).keys().value();
 
 var mergeNames = {
     'stooges': [

lodash/lodash.d.ts

@@ -5637,6 +5637,13 @@ declare module _ {
         keys(object: any): string[];
     }
 
+    interface LoDashObjectWrapper<T> {
+        /**
+        * @see _.keys
+        **/
+        keys(): LoDashArrayWrapper<string>
+    }
+
     //_.mapValues
     interface LoDashStatic {
         /**