add Level Off

Add an Off log level so that loggers can be easily disabled while
allowing other named loggers to use the same synchronized output.

logger.go

@@ -38,6 +38,9 @@ const (
 
 	// Error information about unrecoverable events.
 	Error Level = 5
+
+	// Off disables all logging output.
+	Off Level = 6
 )
 
 // Format is a simple convience type for when formatting is required. When
@@ -96,6 +99,8 @@ func LevelFromString(levelStr string) Level {
 		return Warn
 	case "error":
 		return Error
+	case "off":
+		return Off
 	default:
 		return NoLevel
 	}
@@ -115,6 +120,8 @@ func (l Level) String() string {
 		return "error"
 	case NoLevel:
 		return "none"
+	case Off:
+		return "off"
 	default:
 		return "unknown"
 	}

logger_test.go

@@ -413,6 +413,32 @@ func TestLogger(t *testing.T) {
 		rest = str[dataIdx+1:]
 		assert.Equal(t, "[INFO]  this is another test: production=\"13 beans/day\"\n", rest)
 	})
+
+	t.Run("named logger with disabled parent", func(t *testing.T) {
+		var buf bytes.Buffer
+
+		logger := New(&LoggerOptions{
+			// No name!
+			Output: &buf,
+			Level:  Off,
+		})
+
+		logger.Info("this is test")
+		str := buf.String()
+		if len(str) > 0 {
+			t.Fatal("output from disabled logger:", str)
+		}
+
+		buf.Reset()
+
+		another := logger.Named("sublogger")
+		another.SetLevel(Info)
+		another.Info("this is test")
+		str = buf.String()
+		dataIdx := strings.IndexByte(str, ' ')
+		rest := str[dataIdx+1:]
+		assert.Equal(t, "[INFO]  sublogger: this is test\n", rest)
+	})
 }
 
 func TestLogger_leveledWriter(t *testing.T) {