Add a way to customize the load command to run loadHTML or pass addition...

...al parameters to loadXML

Example:

```php
XmlSecurity::scan($xml, $dom, function ($xml, $dom) {
    return $dom->loadHTML($xml, LIBXML_HTML_NOIMPLIED | LIBXML_HTML_NODEFDTD | LIBXML_NONET);
});
```
@ezimuel