post-cover: add support for adding caption (#17)

* shown only on single page

usage =>
in page-vars front-matter

cover:
  image: '<image path/url>'
  alt: '<alt text>'
  caption: '<text>'

layouts/_default/single.html

@@ -33,6 +33,7 @@
     {{- else}}
     <img src="{{ .Permalink -}} {{- .Params.cover.image }}" alt="{{ .Params.cover.alt | plainify }}">
     {{- end}}
+    <p>{{.Params.cover.caption}}</p>
   </figure>
   {{- end }}
   {{- if .Params.ShowToc }}