improve performance with accent cache (#326).

autoload/airline/section.vim

@@ -6,6 +6,7 @@ let s:spc = g:airline_symbols.space
 
 function! s:wrap_accent(part, value)
   if exists('a:part.accent')
+    call airline#highlighter#add_accent(a:part.accent)
     return '%#__accent_'.(a:part.accent).'#'.a:value.'%#__restore__#'
   endif
   return a:value