Add support for structured levels of alerts, detections and logging

[DonnchaC]

Currently MVT only supports one level of "detected" events. This binary choice of detected or not detected can be limited as we may want to highlight certain suspicious events requiring manual analysis or review.

We currently log warning for certain events. This however is limited as these warnings will only be visible transiently in the command log and not be stored in JSON for later parsing or analysis.

Started work on this in #431.