Fix installed extension detection in jwt.io.

Author: sebadoom

manifest.json

@@ -14,6 +14,11 @@
       "page": "bg.html"
   },
 
+  "content_scripts": [{
+    "matches": ["https://jwt.io/*"],
+    "js": ["js/check-install.js"]
+  }],
+
   "icons": {
       "16": "img/favicon/favicon-16x16.png",
       "32": "img/favicon/favicon-32x32.png",

src/extension/content-scripts/check-install.js

@@ -0,0 +1,4 @@
+const e = document.getElementById('extension-button');
+if(e) {
+    e.classList.add('is-installed');
+}

webpack.extension-dev.js

@@ -5,7 +5,8 @@ const common = require('./webpack.common.js');
 module.exports = merge(common, {
   entry: {
     index: './src/extension/index.js',
-    webstorage: './src/extension/injected/webstorage.js'
+    webstorage: './src/extension/injected/webstorage.js',
+    'check-install': './src/extension/content-scripts/check-install.js'
   },
   output: {
     filename: '[name].js',