Effect g:enable_spelunker_vim_on_readonly option for nomodifiable

[KSR-Yasuda]

Besides readonly, shouldn't it consider nomodifiable setting?

Some plugins may set only nomodifiable without readonly for no-modification-expected buffers
(e.g. fugitive G log window).

[KSR-Yasuda]

Add &filetype ==# 'qf' check, too.

With quickfix list manage plugins such as vim-qfedit,
quickfix list and location list may be &modifiable.