Improve is_logged_indetection

[tarraschk]

The authenticated scan checks if we are connected based on the presence of some words on the webpage:

• https://github.com/wapiti-scanner/wapiti/blob/master/wapitiCore/net/html.py#L19
• https://github.com/wapiti-scanner/wapiti/blob/master/wapitiCore/net/html.py#L646

This approach is very limited and does not work properly on a lot of websites

Any idea to improve this feature?