expose flask-login token-loader to use a custom token_loader

I need to create a custom token (also to avoid using md5 hash) and want to use all the goodies flask-security brings (it's tied with a web application which uses flask-security)
it's just a matter of exposing the token_loader decorator of flask_login.

an example is the following:

```
security = Security(Flask(__name__))

@security.token_loader
def api_token_loader(token):
    try:
        if token is not None:
            user = user_datastore.find_user(user_key=token)
            if user is not None:
                return user
    except:
        pass
    return AnonymousUser()
```

and the user token can be generated any way we desire (i.e. resorting to our favourite RNG and hashing algo)