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.

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*