Implement TextUnmarshaller interface for Level type

Since the implementation of the TextMarshaller interface
we could not unmarshal previously json marshalled Level value.
Fixes #873

logrus.go

@@ -14,24 +14,11 @@ type Level uint32
 
 // Convert the Level to a string. E.g. PanicLevel becomes "panic".
 func (level Level) String() string {
-	switch level {
-	case TraceLevel:
-		return "trace"
-	case DebugLevel:
-		return "debug"
-	case InfoLevel:
-		return "info"
-	case WarnLevel:
-		return "warning"
-	case ErrorLevel:
-		return "error"
-	case FatalLevel:
-		return "fatal"
-	case PanicLevel:
-		return "panic"
+	if b, err := level.MarshalText(); err == nil {
+		return string(b)
+	} else {
+		return "unknown"
 	}
-
-	return "unknown"
 }
 
 // ParseLevel takes a string level and returns the Logrus log level constant.
@@ -69,6 +56,27 @@ func (level *Level) UnmarshalText(text []byte) error {
 	return nil
 }
 
+func (level Level) MarshalText() ([]byte, error) {
+	switch level {
+	case TraceLevel:
+		return []byte("trace"), nil
+	case DebugLevel:
+		return []byte("debug"), nil
+	case InfoLevel:
+		return []byte("info"), nil
+	case WarnLevel:
+		return []byte("warning"), nil
+	case ErrorLevel:
+		return []byte("error"), nil
+	case FatalLevel:
+		return []byte("fatal"), nil
+	case PanicLevel:
+		return []byte("panic"), nil
+	}
+
+	return nil, fmt.Errorf("not a valid lorus level %q", level)
+}
+
 // A constant exposing all logging levels
 var AllLevels = []Level{
 	PanicLevel,