Parse Markdown in summary in page frontmatter

It was impossible to use Markdown in the summary variable in the page
frontmatter. Markdown inside this variable is now parsed.

See #23

layouts/_default/summary.html

@@ -5,7 +5,7 @@ <h1 class="catalogue-title">{{ .Title }}</h1>
         <div class="catalogue-line"></div>
 
         <p>
-            {{ if isset .Params "summary" }}{{ .Params.Summary }}{{ else }}{{ .Summary }}{{ end }}
+            {{ if isset .Params "summary" }}{{ .Params.Summary | markdownify }}{{ else }}{{ .Summary }}{{ end }}
         </p>
     </div>
 </a>