Expose some buffer related functions

This enables to customize the :Buffers commands extensively.

autoload/fzf/vim.vim

@@ -472,7 +472,7 @@ endfunction
 function! s:all_files()
   return fzf#vim#_uniq(map(
     \ filter([expand('%')], 'len(v:val)')
-    \   + filter(map(s:buflisted_sorted(), 'bufname(v:val)'), 'len(v:val)')
+    \   + filter(map(fzf#vim#_buflisted_sorted(), 'bufname(v:val)'), 'len(v:val)')
     \   + filter(copy(v:oldfiles), "filereadable(fnamemodify(v:val, ':p'))"),
     \ 'fnamemodify(v:val, ":~:.")'))
 endfunction
@@ -621,7 +621,7 @@ function! s:bufopen(lines)
   execute 'buffer' b
 endfunction
 
-function! s:format_buffer(b)
+function! fzf#vim#_format_buffer(b)
   let name = bufname(a:b)
   let line = exists('*getbufinfo') ? getbufinfo(a:b)[0]['lnum'] : 0
   let name = empty(name) ? '[No Name]' : fnamemodify(name, ":p:~:.")
@@ -640,15 +640,15 @@ function! s:sort_buffers(...)
   return b1 < b2 ? 1 : -1
 endfunction
 
-function! s:buflisted_sorted()
+function! fzf#vim#_buflisted_sorted()
   return sort(s:buflisted(), 's:sort_buffers')
 endfunction
 
 function! fzf#vim#buffers(...)
   let [query, args] = (a:0 && type(a:1) == type('')) ?
         \ [a:1, a:000[1:]] : ['', a:000]
   return s:fzf('buffers', {
-  \ 'source':  map(s:buflisted_sorted(), 's:format_buffer(v:val)'),
+  \ 'source':  map(fzf#vim#_buflisted_sorted(), 'fzf#vim#_format_buffer(v:val)'),
   \ 'sink*':   s:function('s:bufopen'),
   \ 'options': ['+m', '-x', '--tiebreak=index', '--header-lines=1', '--ansi', '-d', '\t', '--with-nth', '2..', '-n', '2,1..2', '--prompt', 'Buf> ', '--query', query]
   \}, args)