curies and RTL scripts (arabic, hebrew, ...) (PR#8032)

hi,
lately ICANN has been testing IDNs. separators proved to be tricky in
RTL scripts.

http://idn.icann.org/Talk:IDNwiki#RTL_scripts_URL_directionality_problem_.28arabic_for_instance.29

since CURIEs are dependent on a separator (":" for instance) maye a
section in the specs should provide directions for RTL curies.

thank you.