Add AGPL as a license option

[mhauru]

I think cookie should add a copyleft license as an option. I've opted for AGPLv3+, because

1. It is like GPLv3, but also covers the nowadays important case of serving your software over a network, without introducing any real downsides compared to GPLv3. The two are compatible.
2. The "or any later version" clauses avoids things like the mess of GPLv3 and GPLv2 being incompatible.