feature request: add a `exercism restart` command

## description

Add a command to "revert to exercise start".

Just like in the Web UI:

My first thought is an use like this:
```bash
exercism restart --exercise=freelancer-rates --track=javascript
```

Maybe ask for a confirmation before getting rid of user's files (and `--force` option to not ask for confirmation).

## use case

I downloaded an exercise a few days ago but only worked on it today and submitted my solution.

After that I went to the Web UI and noticed the exercise was updated (the requirements/functions to be implemented were changed).

I wanted to get the default boilerplate to restart the exercise from scratch and wasn't able to get it via CLI.

(fortunately @SleeplessByte showed to me how to do it via web UI :heart:)