config: Load all label settings

Until now only the first occurrence of a label type was loaded.

Fixes: https://todo.xenrox.net/~xenrox/ntfy-alertmanager/10

config.go

@@ -85,9 +85,7 @@ func readConfig(path string) (*config, error) {
 
 		labels := make(map[string]labelConfig)
 		for _, labelName := range config.labels.Order {
-			labelDir := labelsDir.Children.Get(labelName)
-
-			if labelDir != nil {
+			for _, labelDir := range labelsDir.Children.GetAll(labelName) {
 				labelConfig := new(labelConfig)
 				var name string