Cleaning allow rel nofollow

See commit messages:
>Bugfix: XHTML Sanitizer did not add nofollow attribute to links
>While the filter actually did add the attribute, the tag attribute
>filter removed it afterwards. Now it does not and we have a test.

>Feature: Add 'add_target_blank' filter to Cleaner
>Currently, it is not used anyhwere but can be added to cleaner settings
>by plugins. Possibly a good default for the description?