syntax match Conceal "||.*||" conceal cchar=?

let s:spoilered = 0

function! ToggleSpoiler()
  if s:spoilered
    hi Conceal guibg=#8087a2 guifg=#8087a2
    let s:spoilered = 0
  else
    hi Conceal guibg=none
    let s:spoilered = 1
  endif
endfunction

nnoremap <F11> :call ToggleSpoiler()<CR>
inoremap <F11> <ESC>:call ToggleSpoiler()<CR>a