add comment system

layouts/partials/comments/utterances.html

@@ -0,0 +1,37 @@
+<script>
+  (function () {
+    const origin = 'https://utteranc.es';
+    const themeMapper = Theme.getThemeMapper('github-light', 'github-dark');
+    const initTheme = themeMapper[Theme.visualState];
+
+    let script = document.createElement('script');
+    script.src = 'https://utteranc.es/client.js';
+    script.setAttribute('repo', '{{ .Site.Params.comments.utterances.repo }}');
+    script.setAttribute('issue-term', '{{ .Site.Params.comments.utterances.issue_term }}');
+    script.setAttribute('theme', initTheme);
+    script.crossOrigin = 'anonymous';
+    script.async = true;
+
+    const $footer = document.querySelector('footer');
+    $footer.insertAdjacentElement('beforebegin', script);
+
+    addEventListener('message', (event) => {
+      let newTheme;
+
+      {{/*
+        Credit to <https://github.com/utterance/utterances/issues/170#issuecomment-594036347>
+      */}}
+      if (event.source === window && event.data && event.data.id === Theme.ID) {
+        newTheme = themeMapper[Theme.visualState];
+
+        const message = {
+          type: 'set-theme',
+          theme: newTheme
+        };
+
+        const utterances = document.querySelector('.utterances-frame').contentWindow;
+        utterances.postMessage(message, origin);
+      }
+    });
+  })();
+</script>