Add --select and --ignore CLI flags for rule filtering

[jmerkow]

Closes #2.

Adds --select and --ignore flags so rules can be toggled from the CLI without a pyproject.toml.

• --select sets the active rules; --ignore removes from it (ruff/flake8 semantics)
• CLI flags override [tool.slopcop] config
• Unknown rule IDs error out before scanning
• Reuses the existing Config.exclude path, so the engine's rule-filtering logic is unchanged
• Includes tests and a README note

[cm2435]

LGTM! thanks for the PR :)

if you want to go further and add some unit test coverage for the flags on the CLI that'd be great. else I'll do it