Add DuckDNS support

[TheDanielDoyle]

Add DuckDNS support.

See API documentation.

https://gist.github.com/taichikuji/6f4183c0af1f4a29e345b60910666468#file-readme-md

[grab-a-byte]

Just to add to the above, the official spec for DuckDNS has better documentation and updating can be done from a single URL (with a csv of domains) However may be worth keeping 1 at a time in case of failures.

Documentation for DuckDNS is here :- https://www.duckdns.org/spec.jsp