Blacklist startify for [noperm]

Startify unfortunately does not set the buftype option, so the
`[noperm]` would be shown, since the buffer is not really a file.
(see mhinz/vim-startify#324)

So allow to blacklist startify from this setting. To do so, the logic
from the tabline extension was refactored out into a separate function
that returns true, if the buffer matches
`g:airline#extensions#tabline#ignore_bufadd_pat` and that function is
then reused for the readonly() function as well.

autoload/airline/parts.vim

@@ -88,7 +88,7 @@ endfunction
 function! airline#parts#readonly()
   " only consider regular buffers (e.g. ones that represent actual files, 
   " but not special ones like e.g. NERDTree)
-  if !empty(&buftype)
+  if !empty(&buftype) || airline#util#ignore_buf(bufname('%'))
     return ''
   endif
   if &readonly && !filereadable(bufname('%'))