Add WriterLevel() function to the logger

This commit adds a variant of the logger's Writer() function that
accepts a log level. When the variant is used, any messages written to
the returned pipe will be written with the provided level. The original
Writer() function uses the logger's Print() method as it always has.

writer.go

@@ -7,18 +7,40 @@ import (
 )
 
 func (logger *Logger) Writer() *io.PipeWriter {
+	return logger.WriterLevel(255)
+}
+
+func (logger *Logger) WriterLevel(level Level) *io.PipeWriter {
 	reader, writer := io.Pipe()
 
-	go logger.writerScanner(reader)
+	var printFunc func(args ...interface{})
+	switch level {
+	case DebugLevel:
+		printFunc = logger.Debug
+	case InfoLevel:
+		printFunc = logger.Info
+	case WarnLevel:
+		printFunc = logger.Warn
+	case ErrorLevel:
+		printFunc = logger.Error
+	case FatalLevel:
+		printFunc = logger.Fatal
+	case PanicLevel:
+		printFunc = logger.Panic
+	default:
+		printFunc = logger.Print
+	}
+
+	go logger.writerScanner(reader, printFunc)
 	runtime.SetFinalizer(writer, writerFinalizer)
 
 	return writer
 }
 
-func (logger *Logger) writerScanner(reader *io.PipeReader) {
+func (logger *Logger) writerScanner(reader *io.PipeReader, printFunc func(args ...interface{})) {
 	scanner := bufio.NewScanner(reader)
 	for scanner.Scan() {
-		logger.Print(scanner.Text())
+		printFunc(scanner.Text())
 	}
 	if err := scanner.Err(); err != nil {
 		logger.Errorf("Error while reading from Writer: %s", err)