Support extract option

Support [**extract** option](http://www.jshint.com/blog/new-in-jshint-dec-2013/) which has been introduced at jshint 2.4.0.

After this change, we can specify extract option such like this:

```js
// Project configuration.
grunt.initConfig({
  jshint: {
    options: {
        extract: 'auto'
    }
  }
});