Fixed #7261 -- support for __html__ for library interoperability

The idea is that if an object implements __html__ which returns a string this is
used as HTML representation (eg: on escaping). If the object is a str or unicode
subclass and returns itself the object is a safe string type.

This is an updated patch based on jbalogh and ivank patches.
Merge pull request #1744 from unaizalakain/ticket_7261

Fixed #7261 -- support for __html__ for library interoperability