fix($medium-zoom): not updating on component update (#994)

* ⬆️ upgrade to medium-zoom@1.0.2

* 🐛 fix medium-zoom not updating on component update

* ⬇️ downgrade to medium-zoom@0.4.0 to fix serverside rendering bug

* 👌 use arrow function 'this' and outsource updateZoom to function

packages/@vuepress/plugin-medium-zoom/mixin.js

@@ -4,9 +4,24 @@ import './style.css'
 import zoom from 'medium-zoom'
 
 export default {
+  data: () => ({ zoom: null }),
+
   mounted () {
-    setTimeout(() => {
-      zoom(SELECTOR)
-    }, 1000)
+    this.updateZoom()
+  },
+
+  updated () {
+    this.updateZoom()
+  },
+
+  methods: {
+    updateZoom () {
+      setTimeout(() => {
+        if (this.zoom) {
+          this.zoom.detach()
+        }
+        this.zoom = zoom(SELECTOR)
+      }, 1000)
+    }
   }
 }