Add features from README

hooks.go

@@ -0,0 +1,28 @@
+package logrus
+
+type Hook interface {
+	Levels() []LevelType
+	Fire(*Entry) error
+}
+
+type levelHooks map[LevelType][]Hook
+
+func (hooks levelHooks) Add(hook Hook) {
+	for _, level := range hook.Levels() {
+		if _, ok := hooks[level]; !ok {
+			hooks[level] = make([]Hook, 0, 1)
+		}
+
+		hooks[level] = append(hooks[level], hook)
+	}
+}
+
+func (hooks levelHooks) Fire(level LevelType, entry *Entry) error {
+	for _, hook := range hooks[level] {
+		if err := hook.Fire(entry); err != nil {
+			return err
+		}
+	}
+
+	return nil
+}