Feature Reqest: ICU (icu::PluralFormat)

First time gh-issuer, long time `stringi` user and zealot :-)

I was thinking about implementing http://www.icu-project.org/apiref/icu4c/classicu_1_1PluralFormat.html outside of `stringi` but was wondering if it has a place in `stringi` instead? I have no pressing need for it but have been in a convo with other who do (and, while https://github.com/hrbrmstr/pluralize is a stop-gap for it, a speedy, robust `stringi` function family would be much better).

Not sure I have the time in the short-term to work on this but wanted to gauge whether it was a possibility before forking and PRing later or if this was already being worked on.