Add vim-rufo extension support to section z.

Displays whether you have the ruby formatter enabled or not for the
session you are in.

Enable with:
let g:airline#extensions#rufo#enabled = 1

Configure with:
let g:airline#extensions#rufo#symbol = '💎'

autoload/airline/extensions.vim

@@ -500,6 +500,11 @@ function! airline#extensions#load()
     call add(s:loaded_ext, 'omnisharp')
   endif
 
+  if (get(g:, 'airline#extensions#rufo#enabled', 0) && get(g:, 'rufo_loaded', 0))
+    call airline#extensions#rufo#init(s:ext)
+    call add(s:loaded_ext, 'rufo')
+  endif
+
 endfunction
 
 function! airline#extensions#get_loaded_extensions()