add inactive statusline funcrefs into the pipeline.

autoload/airline/extensions.vim

@@ -8,6 +8,9 @@ endfunction
 function! s:ext.add_statusline_funcref(function) dict
   call airline#add_statusline_funcref(a:function)
 endfunction
+function! s:ext.add_inactive_statusline_func(name) dict
+  call airline#add_inactive_statusline_func(a:name)
+endfunction
 
 let s:script_path = expand('<sfile>:p:h')