feat: browser object is now accessible in Vue templates by using $browser (Kocal#357)

This will help to have easy access from the template.

```vue
    <button>
      {{ $browser.i18n.getMessage('findPlayer') }}
    </button>
```

Author: 777genius

template/src/popup/popup.js

@@ -1,10 +1,11 @@
-{{#if mozillaPolyfill}}global.browser = require('webextension-polyfill'){{/if}}
-
 import Vue from 'vue'
 import App from './App'
 {{#if store}}import store from '../store'{{/if}}
 {{#if router}}import router from './router'{{/if}}
 
+{{#if mozillaPolyfill}}global.browser = require('webextension-polyfill')
+Vue.prototype.$browser = global.browser{{/if}}
+
 /* eslint-disable no-new */
 new Vue({
   el: '#app',