Add support for post descriptions

Adds support for post descriptions, i.e. subtitles. These are presented
in an italicized h4 header under the post title if present in the Hugo
post configuration.

Author: Joshua Boniface

assets/css/components/_post.scss

@@ -2,6 +2,11 @@
   color: $lightest-color;
 }
 
+.post-description {
+  color: $lightest-color;
+  font-style: italic;
+}
+
 .post-meta > div {
   display: flex;
   align-items: center;

layouts/_default/single.html

@@ -2,6 +2,9 @@
   <article class="post">
     <header class="post-header">
       <h1 class ="post-title">{{ .Title }}</h1>
+      {{- if .Description }}
+      <h4 class ="post-description">{{ .Description }}</h4>
+      {{- end }}
       {{- if ne .Type "page" }}
       <div class="post-meta">
         <div>