Add ability to ouput results as JSON

[absolute-heike]

This adds the --outputJSON parameter so the resulting JSON can be used locally or on CI for further automations.

Example usage: danger local --outputJSON

STDOUT Output

{
  "fails": [],
  "warnings": [],
  "messages": [    
    {
      "message": "foobar"
    }
  ],
  "markdowns": []
}

[orta]

This seems reasonable to me, unsure what's up with circle there