Add support for IP addresses

[freekmurze]

This package should also work on IP address basis.

Let's add two config values:

• auto_allow_ips: can be set to an array of IP address which automatically should have access to the app.
• strict_mode: only the IP addresses in auto_allow_ips are allow to view the site. Access via url should be disabled.

Maybe we can find better names for these new config values.

Of course the new functionality should have tests.