feature request: add `exercism update` command

referencing #802

Instead of having to navigate to the exercise and click the "This exercise has been updated" banner then click the Update button, can we get a cli subcommand to do it?
```sh
exercism update --track=track --exercise=slug
```

And/or, a new option for the download command:
```sh
exercism download --track=track --exercise=slug --update
```
that would apply any updates prior to downloading.