Add single post image alt text, if present.

The current template does not allow for an alt text to the single layout. This small change adds the alt text if present in the page front matter.

layouts/partials/single_post.html

@@ -14,7 +14,7 @@
     {{ end }}
     {{ if .Params.Image }}
         <figure style="margin: 0">
-            <img src="{{ .Params.Image }}" alt="" />
+            <img src="{{ .Params.Image }}" alt="{{ .Params.Alt }}" />
             {{ if .Params.ImageCaption }}
                 <figcaption>
                     <p>{{ .Params.ImageCaption | markdownify }}</p>