post: display the postimage if it exists

Co-Authored-by: iGor milhit <igor@milhit.ch>

assets/scss/_post.scss

@@ -5,13 +5,16 @@
 ///////////////////
 
 .post-header {
-  margin: 1rem 0 .5rem 0;
-
   h1 {
     margin: 0;
   }
 }
 
+.post-image {
+  width: 100%;
+  border-radius: .2rem;
+}
+
 .post-info {
   font-size: $font-size-small;
   color: $snow-storm-light-grey;

layouts/_default/single.html

@@ -1,5 +1,10 @@
 {{ define "main" }}
   <header class="post-header">
+    {{ if .Params.postimage  -}}
+    <img class="post-image"
+         src="/images/{{ .Params.postimage }}"
+         alt="{{ .Params.postimagedescription }}">
+    {{ end }}
     <h1 id="title">{{ .Title }}</h1>
     {{- partial "post-info.html" . -}}
   </header>