Add 'woocommerce_sort_countries' filter

Make checkout countries sorting optional.

Use case:

I have used existing woocommerce_countries filter to redefine the country list placing the most used countries first. However, woocommerce insists on sorting the countries.

So with this filter I can optionally fix this by simply adding:

```php
                add_filter( 'woocommerce_sort_countries', '__return_false' );