add "skip_message"results field to database

[ejaronne]

to accommodate older InSpec profiles lacking in good coding practices.

e.g., if the control code logic simply is: "only_if { false }", then the output could look like this:

      "results": [
        {
          "status": "skipped",
          "code_desc": "Operating System Detection",
          "run_time": 0.000051034,
          "start_time": "2020-08-18T23:47:40-04:00",
          "resource": "Operating System Detection",
          "skip_message": "Skipped control due to only_if condition."
        }

Otherwise, the results won't load, stating error "There are no evaluations in this file"