Feature request: support success message

Apologies if this was already suggested. If you wanted to show a success message as well as a failure message:

```js
validator('presence', {
presence: true,
message: 'this is not right',
successMessage: 'this looks good!'
}),
```

```hbs
{{#if (v-get user "prop" "isInvalid")}}
{{v-get user "prop" "message"}}
{{else}}
{{v-get user "prop" "successMessage"}}
{{/if}}
```