Create example for using DNS01 challenge with Duckdns

[cameronelliott]

I'm not sure if you think this is good or not for the project, but I'm submitting it just in case you think it's useful.

It needs a tiny bit of docs, but I wanted to ask if you prefer comments at the top of the new .rs file, or a readme under /examples? (If you want to merge this...)

This PR depends upon #53 , and will not work without as-is those small changes.