Add a means to reset an exercise to the current state (source and tests)

A had a student recently who asked this question:

> I've been [stuck] on this exercise because I don't know how the original exercise looked like to change the names accordingly.
>
> Please how can I get the original exercise without my modifications? I want to start from scratch but can't get the exercise.

Sometimes the student gets themselves so stuck that they cannot really continue without being able to restart. It would be ideal if I could have them do something like this:

```sh
$ exercism reset --exercise=some-exercise --track=java
```

It can either clear out the existing directories or just copy over the files that are templates / tests. The point is to give the student a fresh start on the exercise!