highligthing: Enable cacheing only when config is set.

This enables the highlighting caching only when the variable
g:airline_highlighting_cache is set to 1

Should make airline faster and more performant, because we can save a
lot of expensive C core calls. However, when redefining highlighting
groups, it might not correctly reset the cache.

plugin/airline.vim

@@ -142,6 +142,7 @@ function! s:airline_refresh()
     let nomodeline = '<nomodeline>'
   endif
   exe printf("silent doautocmd %s User AirlineBeforeRefresh", nomodeline)
+  call airline#highlighter#reset_hlcache()
   call airline#load_theme()
   call airline#update_statusline()
 endfunction