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 
The only option I see atm is to fall back to an xml parser in case php 5.3 and scanHtml is being used. This might yield weird results though so I'm unsure if this is the way to go. Or maybe throw an exception if its used on PHP 5.3 and tell the user to use loadXml instead?