pkg/log: ability for debug logs (#1689)

* pkg/log: ability for debug logs

When the debug plugin is enabled all log.Debug calls will print to
standard; if not there are a noop (almost).

The log package wraps some standard log functions as well, so just
replacing "log" with "plugin/pkg/log" should be enough to use this
package.

* docs

* Add docs

* lint

* Test fallthrough to log pkg as well

* simple package - up test coverage

* add other log levels as well

* update docs

plugin/pkg/log/log_test.go

@@ -0,0 +1,61 @@
+package log
+
+import (
+	"bytes"
+	golog "log"
+	"strings"
+	"testing"
+)
+
+func TestDebug(t *testing.T) {
+	var f bytes.Buffer
+	golog.SetOutput(&f)
+
+	// D == false
+	Debug("debug")
+	if x := f.String(); x != "" {
+		t.Errorf("Expected no debug logs, got %s", x)
+	}
+
+	D = true
+	Debug("debug")
+	if x := f.String(); !strings.Contains(x, debug+"debug") {
+		t.Errorf("Expected debug log to be %s, got %s", debug+"debug", x)
+	}
+}
+
+func TestDebugx(t *testing.T) {
+	var f bytes.Buffer
+	golog.SetOutput(&f)
+
+	D = true
+
+	Debugf("%s", "debug")
+	if x := f.String(); !strings.Contains(x, debug+"debug") {
+		t.Errorf("Expected debug log to be %s, got %s", debug+"debug", x)
+	}
+
+	Debug("debug")
+	if x := f.String(); !strings.Contains(x, debug+"debug") {
+		t.Errorf("Expected debug log to be %s, got %s", debug+"debug", x)
+	}
+}
+
+func TestLevels(t *testing.T) {
+	var f bytes.Buffer
+	const ts = "test"
+	golog.SetOutput(&f)
+
+	Info(ts)
+	if x := f.String(); !strings.Contains(x, info+ts) {
+		t.Errorf("Expected log to be %s, got %s", info+ts, x)
+	}
+	Warning(ts)
+	if x := f.String(); !strings.Contains(x, warning+ts) {
+		t.Errorf("Expected log to be %s, got %s", warning+ts, x)
+	}
+	Error(ts)
+	if x := f.String(); !strings.Contains(x, err+ts) {
+		t.Errorf("Expected log to be %s, got %s", err+ts, x)
+	}
+}