output-json-dns: add new output formats for v2

output-json-dns: add new output formats for v2

This adds two new output formats that permits to reduce
the number of line logged for a dns answer because
actually an event is logged for each answer.
With this patch, only an event that contains all the answers
is logged.

The formats are named 'detailed' and 'grouped'.

The first format provides a list of answers with
the following fields:
- rrname
- rrdata
- ttl
- rdata

The second format provides a list of record data grouped
by their type.

The output below is an example of the formats:

{
  "timestamp": "2017-11-29T10:27:18.148282+0100",
  "flow_id": 268864910185905,
  "in_iface": "wlp2s0",
  "event_type": "dns",
  "src_ip": "192.168.1.254",
  "src_port": 53,
  "dest_ip": "192.168.1.176",
  "dest_port": 52609,
  "proto": "UDP",
  "dns": {
    "type": "answer",
    "id": 3654,
    "rcode": "NOERROR",
    "answers": [
      {
        "rrname": "wordpress.org",
        "rrtype": "A",
        "ttl": 544,
        "rdata": "66.155.40.249"
      },
      {
        "rrname": "wordpress.org",
        "rrtype": "A",
        "ttl": 544,
        "rdata": "66.155.40.250"
      }
    ],
    "grouped": {
      "A": [
        "66.155.40.249",
        "66.155.40.250"
      ]
    }
  }
}