games: add the Gobblet game

I wonder is it appropriate for me to add the Gobblet game in a pull request?

[Gobblet](https://en.wikipedia.org/wiki/Gobblet) is a two player perfect information game.
It is similar to tic-tac-toe, but with a capturing flavour.
Gobblet is significantly harder than tic-tac-toe, yet much simpler than chess, making it a pretty good benchmark for new algorithms.