[FR] Add opts hook for fzf

[jiz4oh]

I have checked the implementation of fzf, it's impossible to extend by the customer.

I have some functions like below:

function! fzf#customized#quickfix(query, fullscreen) abort
  let l:qf = getqflist()
  let l:list = map(l:qf, { _, val -> bufname(val.bufnr). ':'. val.lnum . ':' . val.col . ':'. val.text})
  if !empty(a:query)
    let @/ = a:query
  endif

  let l:actions = {
    \ 'ctrl-t': 'tab split',
    \ 'ctrl-x': 'split',
    \ 'ctrl-v': 'vsplit'
    \}
  let l:dir = getcwd()
  let l:spec = {
                \'dir': l:dir,
                \'sink*': { lines -> fzf#helper#colon_sink(lines, l:actions) },
                \'source': l:list,
                \'options': [
                  \'--expect', join(keys(actions), ','),
                  \'--ansi',
                  \'--prompt', 'Quickfix ',
                  \'--multi', '--bind', 'alt-a:select-all,alt-d:deselect-all',
                  \'--delimiter', ':', '--preview-window', '+{2}-/2'
                \]}

  call fzf#run(fzf#wrap('quickfix', fzf#vim#with_preview(l:spec), a:fullscreen))
endfunction

the fzf#run takes a single
dictionary argument, a spec, and starts fzf process accordingly. depending on the user's choice, the value of this dictionary may be very different. and usually called with fzf#wrap and fzf#vim#with_preview.

could you consider adding new opts for fzf backend like telescope which can let customers customize UI.