fix: Avoid printing result when not using as CLI

[Zaulao]

Avoid printing results to stdout when the tool is not being used as a CLI.

When the tool is used as a lib, it prevents stdout from being written unnecessarily since the value is already returned at the end of the execution of run() and can be handled appropriately by the function that started the execution.

Before:

>>> import dnstwist
>>> result = dnstwist.run(domain='paypal.com', registered=True, format='json')
[
    {
        "dns_a": [
            "151.101.129.21"
        ],
        "dns_mx": [
            "mx1.paypalcorp.com"
        ],
        "dns_ns": [
            "ns1.p57.dynect.net"
        ],
        ...
        ...
        ...
    }
]
>>> print(result[5].get('domain'))
paypalt.com

After:

>>> import dnstwist
>>> result = dnstwist.run(domain='paypal.com', registered=True, format='json')
>>> print(result[5].get('domain'))
paypalt.com

[elceef]

This change breaks the command line tool when --format argument is used effectively muting the output, for example:

$ ./dnstwist.py --format csv github.com

If you need API to not print to standard output, use format='null' as suggested in the documentation.