[vim] Expose buffer variable with the current fzf setup (#828)

Exposing the `b:fzf` variable will be useful to get information about
which command is being executed on the current fzf window. With that,
now, it's possible to use the current command name on the statusline:

```viml
au User FzfStatusLine call <SID>fzf_statusline()

function! s:fzf_statusline()
  let fzf_cmd_name = get(b:fzf, 'name', 'FZF')
  let &l:statusline = '> '.fzf_cmd_name
endfunction
```

plugin/fzf.vim

@@ -201,6 +201,10 @@ function! fzf#wrap(...)
   endfor
   let [name, opts, bang] = args
 
+  if len(name)
+    let opts.name = name
+  end
+
   " Layout: g:fzf_layout (and deprecated g:fzf_height)
   if bang
     for key in s:layout_keys
@@ -521,6 +525,7 @@ function! s:execute_term(dict, command, temps) abort
   let winrest = winrestcmd()
   let pbuf = bufnr('')
   let [ppos, winopts] = s:split(a:dict)
+  let b:fzf = a:dict
   let fzf = { 'buf': bufnr(''), 'pbuf': pbuf, 'ppos': ppos, 'dict': a:dict, 'temps': a:temps,
             \ 'winopts': winopts, 'winrest': winrest, 'lines': &lines,
             \ 'columns': &columns, 'command': a:command }