Implement CLI functionality and enhance README instructions

checkaddy_app/main.py

@@ -1,11 +1,7 @@
 from __future__ import annotations
 
-from .app import CheckAddyApp
-
-
-def main() -> None:
-    CheckAddyApp().run()
+from .cli import main
 
 
 if __name__ == "__main__":
-    main()
+    raise SystemExit(main())