feat(cli): Improve the list command with options, and then some

1.
`rustlings list` should now display more than just the exercise names.
Information such as file paths and exercises statuses should be displayed.
The `--paths` option limits the displayed fields to only the path names; while the `--names`
option limits the displayed fields to only exercise names.
You can also control which exercises are displayed, by using the `--filter` option, or
the `--solved` or `--unsolved` flags.

Some use cases:
- Fetching pending exercise files with the keyword "conversion" to pass to my editor:
```sh
vim $(rustlings list --filter "conversion" --paths --unsolved)
```

- Fetching exercise names with keyword "conversion" to pass to `rustlings run`:
```sh
for exercise in $(rustlings list --filter "conversion" --names)
do
    rustlings run ${exercise}
done
```

2.
This should also fix #465, and will likely fix #585, as well.
That bug mentioned in those issues has to do with the way the `watch` command handler fetches the pending exercises.
Going forward, the least recently updated exercises along with all the other exercises in a pending state are fetched.

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