feat: add --allow flag

[Rustin170506]

ref #493 (comment)

Description

This pull request adds a new flag to tokio-console that allows warning lints to be selectively disabled. Additionally, you can use the value all to disable all warnings.
For example, you can use it like this: following manner: cargo run -- --warn "self-wakes,lost-waker,never-yielded" --allow "self-wakes,never-yielded"

Explanation of Changes

Added an AllowedWarnings enum to the command line interface to represent the allowed warnings. All for disable all warnings. Explicit explicitly indicates which warning can be ignored.

[Rustin170506]

Still working on adding the --allow all support.

[Rustin170506]

Tested locally:

1. panic on main.rs with panic!("{:?}", warnings);
2. Try:

$ cargo run -- --warn "self-wakes" --allow all
     Running `target/debug/tokio-console --warn self-wakes --allow all`
The application panicked (crashed).
Message:  []
Location: tokio-console/src/main.rs:79

3. Another one:

$ cargo run -- --warn "self-wakes" --allow ohno
error: invalid value 'ohno' for '--allow <ALLOW_WARNINGS>...': failed to parse warning: unknown warning: ohno

For more information, try '--help'.

4. Comma list:

$ cargo run -- --warn "self-wakes,lost-waker,never-yielded" --allow "self-wakes,never-yielded"
     Running `target/debug/tokio-console --warn self-wakes,lost-waker,never-yielded --allow self-wakes,never-yielded`
The application panicked (crashed).
Message:  [LostWaker]
Location: tokio-console/src/main.rs:79

[Rustin170506]

🔢 Self-check

[hds]

This looks great, just a couple of small things.

[Rustin170506]

🔢 Self-check

[hds]

Looks good. Perhaps just add the comment on AllowedWarnings regarding ValueEnum that Eliza suggested and it's good to go!

[Rustin170506]

Perhaps just add the comment on AllowedWarnings regarding ValueEnum that Eliza suggested and it's good to go!

Added.
Thanks for your review! 💚 💙 💜 💛 ❤️

[hds]

Added.
Thanks for your review! 💚 💙 💜 💛 ❤️

Great, thank you! Would you mind adding a bit of a longer description to this MR (we'll then use that as the commit message).

Ideally 2 paragraphs: the first one explaining what the PR is trying to achieve (why are we making the change). The second paragraph should explain at a high level what is being changed.

This then becomes a really valuable resource for the future to understand why certain changes have been made.
Thanks!

[hawkw]

This looks good to me! Please make sure to update the commit message the way @hds described, and then we'd love to merge this PR!

[Rustin170506]

Great, thank you! Would you mind adding a bit of a longer description to this MR (we'll then use that as the commit message).

Ideally 2 paragraphs: the first one explaining what the PR is trying to achieve (why are we making the change). The second paragraph should explain at a high level what is being changed.

Added! Thanks for your remainder. I will add it to all my PRs.