Pull request: add log writer

Merge in DNS/golibs from add-log-writer to master

Updates AdguardTeam/AdGuardHome#2273.

Squashed commit of the following:

commit 5e9ca32
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Fri Nov 13 15:28:44 2020 +0300

    log: fix comment

commit 2319364
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Fri Nov 13 15:16:21 2020 +0300

    log: add writer returner

log/log.go

@@ -50,6 +50,11 @@ func (t *Timer) LogElapsed(message string, args ...interface{}) {
 	writeLog(level, f.Name(), buf.String(), args...)
 }
 
+// Writer returns the output destination for the default logger.
+func Writer() io.Writer {
+	return log.Writer()
+}
+
 // SetLevel sets logging level
 func SetLevel(level int) {
 	logLevel = level