Add food-chain exercise

There already are implementations of this exercise in [JavaScript](https://github.com/exercism/xjavascript/tree/master/food-chain) and [Ruby](https://github.com/exercism/xruby/tree/master/food-chain).

The test code is similar to the one for the house exercise. All the internals are left to the student.

A solution with `scanl` is probably also feasible here but more difficult than in the house exercise.