fix issue #14, make delete node configurable

src/VueTreeList.vue

@@ -108,6 +108,10 @@
       defaultTreeNodeName: {
         type: String,
         default: 'New tree node'
+      },
+      onDeleteNode: {
+        type: Function,
+        default: (confirm) => {confirm()}
       }
     },
     computed: {
@@ -160,9 +164,8 @@
 
       delNode () {
         const vm = this
-        if (window.confirm('Are you sure?')) {
-          vm.model.remove()
-        }
+        const confirm = () => vm.model.remove()
+        this.onDeleteNode(confirm)
       },
 
       setEditable () {