Make `require_login` redirect location configurable?

Currently:

```ruby
def require_login
  unless signed_in?
    flash.notice = Monban.config.sign_in_notice
    redirect_to controller: '/sessions', action: 'new'
  end
end
```

This is an issue in "SuperMonban" because the controller is namespaced. Additionally, you might actually want to redirect to your sign up controller - particularly if your sessions are long-lived (it's more likely you are a new user than someone needing to sign in).

Perhaps a configurable string could be used if provided, but the current behavior would be the default?