Update HtmlEditorConfig.php

Added more options to configuration of tinyMCE. 
Need this to using URLs absolute to another area from my site.

_config.php

HtmlEditorConfig::get('cms')->setOption('verify_html', 'false'); // turn off html validation
HtmlEditorConfig::get('cms')->setOption('relative_urls', 'false'); // turn off html validation
HtmlEditorConfig::get('cms')->setOption('remove_script_host', 'false'); // turn off html validation
HtmlEditorConfig::get('cms')->setOption('document_base_url', Director::baseURL()); // set document base url same like baseURL