[fzf#vim#buffers] Allow users to pass a custom list of buffer numbers

Introducing a global configuration variable may seem easier but this
method is more flexible in that it allows you to define multiple
variations of the command without having to repeatedly setting and
unsetting the variable.

  function! MyBuffers()
    return filter(range(1, bufnr('$')), 'buflisted(v:val) && getbufvar(v:val, "&filetype") != "qf"')
  endfunction

  command! -bar -bang Buffers call fzf#vim#buffers(MyBuffers(), fzf#vim#with_preview({'placeholder': '{1}'}), <bang>0)

Close #831
Close #393

autoload/fzf/vim.vim

@@ -807,10 +807,16 @@ function! fzf#vim#_buflisted_sorted()
   return sort(s:buflisted(), 's:sort_buffers')
 endfunction
 
+" [query (string)], [bufnrs (list)], [spec (dict)], [fullscreen (bool)]
 function! fzf#vim#buffers(...)
   let [query, args] = (a:0 && type(a:1) == type('')) ?
         \ [a:1, a:000[1:]] : ['', a:000]
-  let sorted = fzf#vim#_buflisted_sorted()
+  if len(args) && type(args[0]) == s:TYPE.list
+    let [buffers; args] = args
+  else
+    let buffers = s:buflisted()
+  endif
+  let sorted = sort(buffers, 's:sort_buffers')
   let header_lines = '--header-lines=' . (bufnr('') == get(sorted, 0, 0) ? 1 : 0)
   let tabstop = len(max(sorted)) >= 4 ? 9 : 8
   return s:fzf('buffers', {