Introduce $piglatin_domains and piglatin_domains filter

Fixes #4. Adds both the `$piglatin_domains` global variable and a `piglatin_domains` filter.

By default, allows all domains unless either of the above is used, in which case, only uses domains set in that variable. To use, add something like this to your `wp-config.php`:

```php
$piglatin_domains = [
	'myplugin' => true,
];
```