disable markdown by default

src/config.ts

@@ -83,9 +83,15 @@ export class Instance {
   })
   resolvedMessage: string;
 
+  @SchemaProperty({
+    description: '使用 markdown 语法。',
+    default: false,
+  })
+  markdown: boolean;
+
   formatAlert(dto: AlertDto) {
     const { alerts } = dto;
-    let text = `${segment('markdown')}# ${this.title}`;
+    let text = `${this.markdown ? segment('markdown') : ''}# ${this.title}`;
 
     for (const alert of alerts) {
       text += `\n\n## ${alert.labels.alertname}\n\n`;