Implement additional section processing

When IBDNS returns resource records that require additional section processing, such as MX or NS records, it does not attempt to add A or AAAA addresses in the response’s additional section.

Additionally, the following should be configurable:

• whether additional section processing is performed at all (or maybe do it only for a subset of RRTYPEs that require it: one may wish to only do it for NS but not MX, for example);
• which resource record types are searched for in the database during additional section processing: RFC 1035 §3.3 only mentions the A type and RFC 3596 §3 updates that in order to include A and AAAA. It should be possible to do additional section processing with A only, AAAA only or both A and AAAA.