Add DuckDNS as a free DNS provider option (#124)

DuckDNS's API is smaller than the other providers' (no list/read API,
no arbitrary sub-records, one A/AAAA + one TXT record per domain), so
domains are entered by the operator instead of auto-discovered, and
getRecords reads from public DNS since there's nothing else to query.
Documented as a free option in the README and DNS provider docs.

nodejs/models/dns_provider.js

@@ -13,6 +13,7 @@ const providers = {
 	Cloudflare: require('./dns_provider/cloudflare'),
 	DigitalOcean: require('./dns_provider/digitalocean'),
 	PorkBun: require('./dns_provider/porkbun'),
+	DuckDns: require('./dns_provider/duckdns'),
 };
 
 class Domain extends Table{