Concept exercise documents

As discussed on the [forum](http://forum.exercism.org/t/concept-exercise-files/13289/7), we probably want to modify `configlet create --concept-exercise` to also add `.meta/design.md`

There was also some inconclusive discussion around:
- Aso add `.docs/hints.md`.
- Get `configlet lint` to warn if these documents are missing (though that hits the problem that many old exercises lack them).