[Feature request]: Add --preview to :Snippets command to ease snippet selection

- [x] I have read through https://github.com/junegunn/fzf.vim/blob/master/README.md
- [x] I have read through https://github.com/junegunn/fzf/blob/master/README-VIM.md
- [x] I have read through the manual page of fzf (`man fzf`)
- [x] I have searched through the existing issues

It works with :Files :Ag and :Rg. That's awesome, thank you so much!
Adding it to :Snippets should be easy. The customization of the :Files command is described here:
https://github.com/junegunn/fzf.vim/blob/master/README.md#example-customizing-files-command

The default :Snippets command is:
```
command! -bar -bang Snippets call fzf#vim#snippets(0)
```
How do I add `--preview` if there is no `` in `fzf#vim#snippets(...)`?

I tried this but i need some help to get it right
```
command! -bang -complete=dir Snippets
\ call fzf#vim#snippets({'options': ['--layout=reverse', '--info=inline', '--preview', 'cat {}']}, 0)
```