added webmanifest support

templates/index.html.j2

@@ -5,6 +5,9 @@
   <meta charset="UTF-8">
   <meta name="viewport" content="width=device-width, initial-scale=1.0">
   <title>{{ title }}</title>
+  {%- if webmanifest %}
+  <link rel="manifest" href="/manifest.json">
+  {%- endif %}
   <link rel="icon" type="image/x-icon" href="{{ favicon }}">
   <link rel="stylesheet" href="{{ stylesheet }}">
   {%- if theme %}