Fix json (goss-org#841)

* Added ctx object to all system struct's constructors

The idea behind this commit is to set the stage for adding aditional
logging metadata.

* Changed: Improved logging

* Added new flags `--log-level` and `-l` to be consistent with other goss
flags.
  * `--loglevel` and `-L` will be deprecated in a future release
* Log level accepts any casing, so DEBUG and debug are both valid cli opts
* Add log level support for `add` and `autoadd` commands

* Changed log time format to RFC3339

* Fixed: json result[].successful to match goss 3.X behavior

fixes goss-org#840

outputs/json.go

@@ -56,6 +56,7 @@ func (r Json) Output(w io.Writer, results <-chan []resource.TestResult,
 				skipped++
 			}
 			m := struct2map(testResult)
+			m["successful"] = testResult.Result != resource.FAIL
 			m["summary-line"] = humanizeResult(testResult, false, includeRaw)
 			m["summary-line-compact"] = humanizeResult(testResult, true, includeRaw)
 			m["duration"] = int64(m["duration"].(float64))