Feature request: add Domain Offensive (do.de) FlexDNS

1. Updating do.de FlexDNS

2. Documentation:

Wiki: https://www.do.de/wiki/flexdns-einrichtung/
Example (meant for ddclient):
```
daemon=600
cache=/tmp/ddclient.cache
pid=/var/run/ddclient.pid
use=web, web=checkip.dyndns.com/, web-skip='IP Address'
login=DDNS-KDAAAAA-XYYYY
password=BbbBBbbBBb
protocol=dyndns2
server=ddns.do.de
meinedomain.de
```

cheers!