Generate Pandoc file with correct slug

This patch fixes a bug where the slug of a generated pandoc file will
be `nil` if the title argument passed to the constructor is `nil`.
Instead, the slug should be based on the title chosen earlier in the
constructor.

lib/jekyll-pandoc-multiple-formats/pandoc_file.rb

@@ -46,7 +46,7 @@ module Jekyll
         @title = title or posts.data['title']
       end
 
-      @slug  = Utils.slugify(title)
+      @slug = Utils.slugify(@title)
     end
 
     def path