Add logic to use .Description instead of .Summary

Adds a check for a useDescription param which will check for a
description field in a post and use it instead of the .Summary when
producing list.html. If a description is unavailable and hideSummary is
not true, the summary will show as normal.

layouts/_default/list.html

@@ -72,7 +72,11 @@
       {{- if .Draft }}<sup><span class="entry-isdraft">&nbsp;&nbsp;[draft]</span></sup>{{- end }}
     </h2>
   </header>
-  {{- if (ne (.Param "hideSummary") true) }}
+  {{- if and (.Description) (.Param "useDescription") }}
+  <div class="entry-content">
+    <p>{{ .Description | plainify | htmlUnescape }}</p>
+  </div>
+  {{- else if (ne (.Param "hideSummary") true) }}
   <div class="entry-content">
     <p>{{ .Summary | plainify | htmlUnescape }}{{ if .Truncated }}...{{ end }}</p>
   </div>