driver: Change a variable name to avoid squatting the highlighters' namespace.

Part of issue #329.

zsh-syntax-highlighting.zsh

@@ -81,7 +81,7 @@ _zsh_highlight()
     local highlighter; for highlighter in $ZSH_HIGHLIGHT_HIGHLIGHTERS; do
 
       # eval cache place for current highlighter and prepare it
-      cache_place="_zsh_highlight_${highlighter}_highlighter_cache"
+      cache_place="_zsh_highlight_highlighter_${highlighter}_cache"
       typeset -ga ${cache_place}
 
       # If highlighter needs to be invoked