Feature: Add stats to JSON output

[miketheman]

When using mal in an automated context, using --format json is the likeliest interface for other processes to evaluate the output.

Currently, the JSON output has one top-level key - "Files", which we can traverse to get the RiskLevel or RiskScore, but we have to iterate through each to collect them and calculate a score.

The --stats command currently does not affect the JSON output, only printing to terminal, where it would be preferable to include that information in the JSON output, probably as a top-level "Stats" key, something like:

{
  "Files": {...},
  "Stats": {
    FilesScanned: <int>,
    FilesSkipped: <int>,
    TotalRisks: <int>,
    RiskLevel: <str>,
    RiskPercentage: <float>,
    BehaviorCount: <int>,
    ...
  }
}

I'm not certain on the namespace details yet, but wanted to get the conversation started on whether this is a good idea or not.