Top-level domains selection (-t/--tld flag)

The --tld flag lets the user choose top-level domains.

```
main.py --tld com|org|...
```

Look at the argument parsing in main.py to see how flags are currently handled. The new --tld flag should accept values like 'com' or 'org'. Check how the domain discovery logic uses TLDs and update it to filter based on the provided flag. Run the script with the new flag to verify it works.

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