json: add custom output capability to http eve log

This is the eve-log custom http log output capability changes.  This permits the addition of custom http fields in the json eve log output via configuration contained in the suricata.yaml file.

- PR build: https://buildbot.suricata-ids.org/builders/decanio/builds/16
- PR pcaps: https://buildbot.suricata-ids.org/builders/decanio-pcap/builds/3