Add support for Unicode grapheme cluster input iterators

[tahonermann]

[ruoso]

Interestingly enough, I have been doing a pet project on my own on this very topic...

https://github.com/ruoso/u5e/blob/master/t/006_grapheme_iterator.t.cpp#L16

[tahonermann]

Great, thanks for the link. I'll take a look when I get some time.