Add logout command

[daveliu]

Closes #3.

Adds a logout command that revokes the current server session and removes the stored CLI credentials.

Changes

• Add config::remove_config, which deletes the config file and reports whether one existed.
• Add an authenticated DELETE /my/session request before local credentials are removed.
• Keep the config intact if server-side revocation fails.
• Print a clear message for both the logged-in and not-logged-in cases.
• Document blackcandy logout in the README.

Validation

• cargo fmt --check
• cargo test
• cargo clippy -- -D warnings

[aidewoode]

I think we should also called DELETE /my/session to delete session on server side. otherwise we will have orphan session records