Add $post_type functionality

[terence1990]

This helps with simplifying type scope so users can do this:

    namespace Theme\Models;

    use WPEloquent\Model\Post;

    class Product extends Post {

        protected $post_type = 'product';

    }

// this will return a product automatically
var_dump(Product::first());

[drewjbartlett]

You're the man. Thanks for all your contributions to this.