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 
@ezimuel ping on this.

 > Or maybe throw an exception if its used on PHP 5.3 and tell the user to use loadXml instead? 

Can't really do, as there are parts of the codebase loading DOM nodes that are indeed used also in PHP 5.3 context. Writing an HTML parser is not attracting either, and would just be another huge source of trouble.