Add support to customize issue-term (mmistakes#2022)

Current we hardcoded the `pathname` for `issue-term` when we are using utterances comment provider.

It would be better for let users customize it by setting a `site.comments.utterances.issue_term` in their `_config.yml`.

Author: huan

_includes/comments-providers/utterances.html

@@ -11,10 +11,10 @@
     var script = document.createElement('script');
     script.setAttribute('src', 'https://utteranc.es/client.js');
     script.setAttribute('repo', '{{ site.repository }}');
-    script.setAttribute('issue-term', 'pathname');
+    script.setAttribute('issue-term', '{{ site.comments.utterances.issue_term | default: "pathname" }}');
     script.setAttribute('theme', '{{ site.comments.utterances.theme | default: "github-light" }}');
     script.setAttribute('crossorigin', 'anonymous');
 
     commentContainer.appendChild(script);
   })();
-</script>
+</script>