add danmu filter

docs/mobile.html

@@ -122,10 +122,11 @@
                     danmuku: '/assets/sample/danmuku.xml',
                     speed: 5,
                     maxlength: 50,
-                    margin: [10, 50],
+                    margin: ['10%', 60],
                     opacity: 1,
                     fontSize: 14,
                     synchronousPlayback: false,
+                    filter: (danmu) => danmu.text.length < 20,
                 }),
             ],
             whitelist: ['*'],