Update vanilla theme to handle loading Bergamot

Signed-off-by: Danila Fedorin <danila.fedorin@gmail.com>

layouts/shortcodes/bergamot_preset.html

@@ -1,8 +1,10 @@
 <script>
-    window.Bergamot.presets['{{ .Get "name" }}'] = {
-        rules: '{{ .Inner }}',
-        inputPrompt: '{{ .Get "prompt" }}',
-        query: '{{ .Get "query" }}',
-        renderPreset: '{{ default "default" (.Get "rendering") }}'
-    };
+    window.addEventListener('load', function() {
+        window.Bergamot.presets['{{ .Get "name" }}'] = {
+            rules: '{{ .Inner }}',
+            inputPrompt: '{{ .Get "prompt" }}',
+            query: '{{ .Get "query" }}',
+            renderPreset: '{{ default "default" (.Get "rendering") }}'
+        };
+    });
 </script>