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

[AbdouSeck]

The PR is two-fold:

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.

Sample invocations:

# Display exercises that deal with traits, or enums, or structs.
rustlings list --filter "trait,enum,struct"

# Sample output:
# Name             	Path                                          	Status
# structs1         	exercises/structs/structs1.rs                 	Pending
# structs2         	exercises/structs/structs2.rs                 	Pending
# structs3         	exercises/structs/structs3.rs                 	Pending
# enums1           	exercises/enums/enums1.rs                     	Pending
# enums2           	exercises/enums/enums2.rs                     	Pending
# enums3           	exercises/enums/enums3.rs                     	Pending
# traits1          	exercises/traits/traits1.rs                   	Pending
# traits2          	exercises/traits/traits2.rs                   	Pending

# Display only exercises pending to be solved
rustlings list --unsolved

# Sample output:
# Name             	Path                                          	Status
# structs2         	exercises/structs/structs2.rs                 	Pending
# structs3         	exercises/structs/structs3.rs                 	Pending
# enums1           	exercises/enums/enums1.rs                     	Pending
# enums2           	exercises/enums/enums2.rs                     	Pending
# enums3           	exercises/enums/enums3.rs                     	Pending
# modules1         	exercises/modules/modules1.rs                 	Pending
# modules2         	exercises/modules/modules2.rs                 	Pending
# collections1     	exercises/collections/vec1.rs                 	Pending
# collections2     	exercises/collections/vec2.rs                 	Pending
# collections3     	exercises/collections/hashmap1.rs             	Pending
# collections4     	exercises/collections/hashmap2.rs             	Pending

This portion should also close #281.

Some shell utility use cases:

• Fetching pending exercise file paths with the keyword "conversion" to pass to my editor:

vim $(rustlings list --filter "conversion" --paths --unsolved)

• Fetching exercise names with keyword "conversion" to pass to rustlings run:

for exercise in $(rustlings list --filter "conversion" --names)
do
    rustlings run ${exercise}
done

2. The PR should also fix Unexpected completion #465, and will likely fix as_ref_mut not ran #585, as well. The 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.

@fmoko @jrvidal if you've got some time, I'd love an eye on this. If there is anyone else with some experience on (or interest in) the CLI component, please feel free to let me know what you think.

Thanks,
Abdou

[jrvidal]

Some minor comments, but this looks good @AbdouSeck 🚀