Add utteranc comment plugin (#465)

* add comment plugin utteranc(save message to github issue)

config.yml
    # Enable comment feature. There, should be only one of them.
    comment:
      enable: true
      utteranc:
        repo: ormissia/ormissia.github.io
        issueTerm: title
        theme: github-light

* Update utteranc.html

layouts/_default/single.html

@@ -165,6 +165,9 @@
           <!-- Add valine -->
           {{ else if .valine }}
               {{ partial "valine.html" . }}
+          <!-- Add utteranc -->
+          {{ else if .utteranc }}
+              {{ partial "utteranc.html" . }}
           {{ end }}
         {{ end }}
       {{ end }}

layouts/partials/utteranc.html

@@ -0,0 +1,22 @@
+{{ $repo := site.Params.features.comment.utteranc.repo }}
+{{ $issueTerm := site.Params.features.comment.utteranc.issueTerm }}
+{{ $theme := site.Params.features.comment.utteranc.theme }}
+
+<div id="utteranc_thread"></div>
+<div id="comments" class="comments">
+    <div id="comments-container"></div>
+</div>
+<script type="text/javascript">
+    (function() {
+        var utterances = document.createElement('script');
+        utterances.type = 'text/javascript';
+        utterances.async = true;
+        utterances.setAttribute('repo','{{ $repo }}')
+        utterances.setAttribute('issue-term','{{ $issueTerm }}')
+        utterances.setAttribute('theme','{{ $theme }}')
+        utterances.crossorigin = 'anonymous';
+        utterances.src = 'https://utteranc.es/client.js';
+
+        document.getElementById('comments-container').appendChild(utterances);
+    })();
+</script>