* refine the new dns resolving process
* add a caching mechanism for the resolved IPs, the detached name
  lookup takes place only during 'restart' or 'reload' action, 'start'
  and 'refresh' actions are using an auto-generated backup instead.
* update the readme

Signed-off-by: Dirk Brenken <dev@brenken.org>

* black- and whitelist now supporting domain names as well - the
corresponding IPs (IPv4 & IPv6) will be resolved in a detached
background process and added to the IPsets

Signed-off-by: Dirk Brenken <dev@brenken.org>