怎么通过点击事件关闭Element ui 的Notification 通知
点击打开弹出通知，点击关闭怎么关闭通知？
```
<div>
    <el-button plain @click="open">打开</el-button>
    <el-button @click="close">关闭</el-button>
</div>

methods: {
            open() {
                this.$notify({
                    title: '提示',
                    message: '这是一条不会自动关闭的消息',
                    duration: 0
                });
            },
            close() {
                this.$notify.close()
            }
        }
```
this.$notify 会返回当前 Notification 的实例
```
methods: {
            open() {
                this.instance = this.$notify({
                    title: '提示',
                    message: '这是一条不会自动关闭的消息',
                    duration: 0
                });
            },
            close() {
                this.instance.close()
            }
        }
```