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.

1 files changed, 7 insertions, 0 deletions

diff --git a/tests/fixture/state/info.toml b/tests/fixture/state/info.toml
index 7bfc697..547b3a4 100644
--- a/tests/fixture/state/info.toml
+++ b/tests/fixture/state/info.toml
@@ -9,3 +9,10 @@ name = "pending_test_exercise"
 path = "pending_test_exercise.rs"
 mode = "test"
 hint = """"""
+
+[[exercises]]
+name = "finished_exercise"
+path = "finished_exercise.rs"
+mode = "compile"
+hint = """"""
+