[vim] fzf#run with tmux panes can now return values to the caller

As they're made synchronous with the use of fzf-tmux script

plugin/fzf.vim

@@ -202,7 +202,7 @@ function! s:execute_tmux(dict, command, temps)
   endif
 
   call system(command)
-  call s:callback(a:dict, a:temps)
+  return s:callback(a:dict, a:temps)
 endfunction
 
 function! s:callback(dict, temps)