Allow free commands in --input-cmd

### Requested feature
Sometimes we need to manipulate inputs into something closer to what we want, like cutting a string with cut and then encoding the value in base64.

Exemples:

ffuf --input-cmd "echo 192.168.1.\$FFUF_NUM" -u "http://FUZZ" -mc 200

ffuf --input-cmd "python3 generate_payload.py" -u "https://example.com/FUZZ" -mc 200

ffuf --input-cmd "cat subdomains.txt | grep example" -u "https://FUZZ.com" -mc 200

Thanks.