group option to avoid duplicate

The `findExisting` method only return a flash message with the same message content.
This group option allows to avoid duplicate flash when message content is dynamic.
Can we re-use `unique` and `type` for this?

e.g. change unique to `unique_by` and default it to `'message'`. If you pass in `'type'` we can compare the message type.

The only problem is we use `type` as the css class, so it looks like `alert error` etc. Could get complicated if your message types don't match. But, could be cleaner if it will suit your usecase.