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

...al parameters to loadXML

Example:

```php
$xmlDom = XmlSecurity::scan($xml, $dom, LIBXML_COMPACT);
$htmlDom = XmlSecurity::scanHtml($html, $dom, LIBXML_HTML_NOIMPLIED | LIBXML_HTML_NODEFDTD);
```
@ezimuel 
ATM yes, I'll add a check for libxml version and skip the test like mentioned in the phpunit docs, sec ;)
```php
if (!extension_loaded('mysqli')) {
            $this->markTestSkipped(
              'The MySQLi extension is not available.'
            );
        }
```