Log Message:
textproc/R-stringr: Update to 1.2.0

Notable changes:
- str_match_all() now returns NA if an optional group doesn't match
  (previously it returned ""). This is more consistent with
  str_match() and other match failures.
- In str_replace(), replacement can now be a function that is called
  once for each match and who's return value is used to replace the
  match.
- New str_which() mimics grep().
- A new vignette (vignette("regular-expressions")) describes the
  details of the regular expressions supported by stringr. The main
  vignette (vignette("stringr")) has been updated to give a high-level
  overview of the package.