compile markdown for each request

The problem here is, that the markdown code can't be compiled and
therefore does not get executed when the page is rendered. So this
change directly calls Kramdown to compile the code.
It should be replaced later by a mechanism which compiles the markdown
when the post is loaded.

templates/posts/index.html.slim

@@ -16,6 +16,4 @@ html
             .author=post[:username]
             .date=  post[:written].strftime('%Y-%m-%d')
             .time=  post[:written].strftime('%H-%M')
-          section
-            markdown:
-              "#{post[:content]}"
+          section==Kramdown::Document.new(post[:content]).to_html