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/extensions/tabline.vim

@@ -49,14 +49,13 @@ function! s:update_tabline()
   endif
   let match = expand('<afile>')
   let ignore_bufadd_pat = get(g:, 'airline#extensions#tabline#ignore_bufadd_pat',
-        \ '\c\vgundo|undotree|vimfiler|tagbar|nerd_tree')
+        \ '\c\vgundo|undotree|vimfiler|tagbar|nerd_tree|startify')
   if pumvisible()
     return
   elseif !get(g:, 'airline#extensions#tabline#enabled', 0)
     return
   " return, if buffer matches ignore pattern or is directory (netrw)
-  elseif empty(match)
-        \ || match(match, ignore_bufadd_pat) > -1
+  elseif empty(match) || airline#util#ignore_buf(match)
         \ || isdirectory(expand("<afile>"))
     return
   endif

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('%'))

autoload/airline/util.vim

@@ -96,3 +96,10 @@ function! airline#util#strchars(str)
     return strlen(substitute(a:str, '.', 'a', 'g'))
   endif
 endfunction
+
+function! airline#util#ignore_buf(name)
+  let pat = '\c\v'. get(g:, 'airline#ignore_bufadd_pat', '').
+        \ get(g:, 'airline#extensions#tabline#ignore_bufadd_pat', 
+        \ 'gundo|undotree|vimfiler|tagbar|nerd_tree|startify')
+  return match(a:name, pat) > -1
+endfunction

doc/airline.txt

@@ -324,6 +324,12 @@ section.
   let g:airline_section_b = '%-0.10{getcwd()}'
   let g:airline_section_c = '%t'
 <
+                                                |airline#ignore_bufadd_pat|
+Determines a pattern to ignore a buffer name for various things (e.g. the
+tabline extension) or the read-only check. Default is
+`g:airline#extensions#tabline#ignore_bufadd_pat` (see below) or
+`'gundo|undotree|vimfiler|tagbar|nerd_tree|startify'` if it is unset.
+
 ==============================================================================
 EXTENSIONS                                              *airline-extensions*