Output in JSON and YAML

### Problem

The current implementation only allows text-based output:

```
Difference found: CONNECT_TIMEOUT
my.cnf: 60
mysqld: 30
```

Output should be allowed in a format that is more easily to consume programatically, e.g. JSON or YAML.

### Acceptance Criteria

- [ ] User is able to get output in JSON and YAML.
- [ ] User should be able to specify using a flag, like `--format [json|text|yaml]` or something like that
- [ ] Unit tests
- [ ] Update README

Start by locating the CLI flag handling and the existing text-output path, then review how unit tests are organized. Add coverage for JSON, YAML, and text selection, update the README with the flag usage, and consider the work done when all three formats can be requested successfully.

Written by the indexing model from the issue text.