Tag messages

Hello,

I'm using Django messaging framework in my project, as soon as I enabled it, I started receiving messages from django-allauth. I needed to selectively enable/disable messages from django-allauth. I added extra_tags to all messages and they can be filtered in Django settings, like this:

```python
# Empty collection disables filtering
MESSAGE_FILTER_TAGS = set((
    'account_email_confirm',
    'account_email_send',
))
```

I use the following TEMPLATE_CONTEXT_PROCESSOR to filter messages:
```python
def messages(request):
    """
    Returns filtered 'messages' context variable.

    """
    messages = get_messages(request) or []
    if settings.MESSAGE_FILTER_TAGS:
        messages = [m for m in messages
                    if settings.MESSAGE_FILTER_TAGS.intersection(
                        m.extra_tags.split() if m.extra_tags else ())]
    return {'messages': messages}
```

Is this something that could be useful for a broader community of users of django-allauth?