Merge: contrib: Introduce refund

`refund` is a tool developped for the correction of the course INF2015: Développement de logiciels dans un environnement Agile.

Its purpose is to parse a reclamation sheet submitted by an insurrance client in Json format then to compute the allowed refunds depending on the policy contracted by the client.

Even if this program is not really usefull for the Nit project, it makes a concrete example on how we can use the language in the real world, so I submit it.

Pull-Request: #1313
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>
Reviewed-by: Jean Privat <jean@pryen.org>