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 
5.3 does not allow any parameters to be passed to loadHtml so LIBXML_NONET can not be passed in, see http://php.net/manual/de/domdocument.loadhtml.php

I'm unsure if this can be fixed.