Added ability to type hint a task

Added ability to type hint a Grunt task so that syntax like:

```php
<?php

use League\Grunt\Grunt;

return [
    'hosts' => [
        'production' => 'foo.com'
    ],
    'tasks' => [
        'deploy' => function(Grunt $g) {
            $g->run('ls');
        }
    ],
];
```

Can be used. This gives the ability to autocomplete methods from an IDE, as well as aid in using debuggers.