Expose all levels (for hooks).

hooks/syslog/syslog.go

diff --git a/hooks/syslog/syslog.go b/hooks/syslog/syslog.go
index c59f331..a36e200 100644
--- a/hooks/syslog/syslog.go
+++ b/hooks/syslog/syslog.go
@@ -50,12 +50,5 @@
 }
 
 func (hook *SyslogHook) Levels() []logrus.Level {
-	return []logrus.Level{
-		logrus.PanicLevel,
-		logrus.FatalLevel,
-		logrus.ErrorLevel,
-		logrus.WarnLevel,
-		logrus.InfoLevel,
-		logrus.DebugLevel,
-	}
+	return logrus.AllLevels
 }

logrus.go

diff --git a/logrus.go b/logrus.go
index fa8a7cc..1e9670d 100644
--- a/logrus.go
+++ b/logrus.go
@@ -53,6 +53,16 @@
 	return l, fmt.Errorf("not a valid logrus Level: %q", lvl)
 }
 
+// A constant exposing all logging levels
+var AllLevels = []Level{
+	PanicLevel,
+	FatalLevel,
+	ErrorLevel,
+	WarnLevel,
+	InfoLevel,
+	DebugLevel,
+}
+
 // These are the different logging levels. You can set the logging level to log
 // on your instance of logger, obtained with `logrus.New()`.
 const (