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 }}