stringi_1.1.6

CHANGELOG:

* [WINDOWS SPECIFIC] #270: Strings marked with `latin1` encoding
are now converted internally to UTF-8 using the WINDOWS-1252 codec.
This fixes problems with - among others - displaying the Euro sign.

* [NEW FEATURE] #263: Add support for custom rule-based break iteration,
see `?stri_opts_brkiter`.

* [NEW FEATURE] #267: `omit_na=TRUE` in `stri_sub<-` now ignores missing values
in any of the arguments provided.

* [BUGFIX] fixed unPROTECTed variable names and stack imbalances
as reported by rchk