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.
Wouldn't you want to filter then messages by the group, if it's provided, then perform the same logic, looking for a message with the same content?

```javascript
var messages = _this.messages()

if (_this.group) {
  messages = _.filter(messages, function(m) { m.group == _this.group }) 
}

angular.forEach(messages, function(flashMessage) {
  if (flashMessage.message === _this.message) {
    found = flashMessage
  }
})
```