feat: new methods to check enabled log level

Adding 6 methods on 'exported', 'logger' and 'entry':
- IsDebugEnabled() bool
- IsInfoEnabled() bool
- IsWarnEnabled() bool
- IsErrorEnabled() bool
- IsFatalEnabled() bool
- IsPanicEnabled() bool

Replace duplicated 'if logger.level() >= XxxxLevel' by a call to the new methods in 'logger' and 'entry'

Closes #761

logrus.go

@@ -140,4 +140,11 @@ type FieldLogger interface {
 	Errorln(args ...interface{})
 	Fatalln(args ...interface{})
 	Panicln(args ...interface{})
+
+	IsDebugEnabled() bool
+	IsInfoEnabled() bool
+	IsWarnEnabled() bool
+	IsErrorEnabled() bool
+	IsFatalEnabled() bool
+	IsPanicEnabled() bool
 }