Display an optional message when date is null/invalid/undefined

The default behavior when the helper receives an invalid input is to return **Invalid Date**.

Optionally, from docs we can allow empty dates to be passed but it returns nothing:

> ```{{moment-format '' allow-empty=true} {{!-- --}}```

But it could help a lot in my case if I could define an optional message. I'm currently doing this via a template helper:

```js
export function formattedDate([date]) {
return date ? moment(date).format('DD/MM/YY') : 'indisponível';
}

export default helper(formattedDate);
```

If possible, I'd like to eliminate this helper.

Start with the moment-format helper and the documentation examples for allow-empty, then trace how empty, invalid, and undefined inputs are handled. Done means an optional message can be specified for those inputs while the existing default and allow-empty behavior remain documented and tested.

索引モデルが issue の本文から書いたものです。