Fix #56: tolerance to WARN_GLOBAL option

zsh-syntax-highlighting.zsh

@@ -42,6 +42,8 @@ typeset -ga ZSH_HIGHLIGHT_HIGHLIGHTERS
 # This function is supposed to be called whenever the ZLE state changes.
 _zsh_highlight()
 {
+  setopt localoptions nowarncreateglobal
+
   # Store the previous command return code to restore it whatever happens.
   local ret=$?