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.

autoload/airline/highlighter.vim

@@ -53,7 +53,7 @@ function! airline#highlighter#reset_hlcache()
 endfunction
 
 function! airline#highlighter#get_highlight(group, ...)
-  if has_key(s:hl_groups, a:group)
+  if get(g:, 'airline_highlighting_cache', 0) && has_key(s:hl_groups, a:group)
     return s:hl_groups[a:group]
   else
     let fg = s:get_syn(a:group, 'fg')

doc/airline.txt

@@ -211,6 +211,10 @@ values):
   the same name (in the correct window): >
   let w:airline_skip_empty_sections = 0
 <
+* Caches the changes to the highlighting groups, should therefore be faster.
+  Set this to one, if you experience a sluggish Vim: >
+  let g:airline_highlighting_cache = 0
+<
 ==============================================================================
 COMMANDS                                                  *airline-commands*
 

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