DuckDNS: Add support for typing the full domain.

[ludeeus]

This will allow users to type the domain in various ways:

• example
• example.duckdns.org
• example.duckdns.org:8123
• http://example.duckdns.org
• http://example.duckdns.org:8123
  And it will always use example in the rest of the script.

---

• Created/Updated documentation at /docs/
• Script has validation check of the job.
• Script is tested locally.

[Landrash]

Looks good to me 👍