Display title and subtitle

Also wrap the content in an article for semantic formatting

tufte.css requires that content is wrapped in an <article> and at least
one <section>. There's no way of determining new semantic sections so
there is only one.

src/pages/articles/show_page.cr

@@ -2,6 +2,14 @@ class Articles::ShowPage < MainLayout
   needs page : Page
 
   def content
-    mount PageContent, page: page
+    h1 page.title
+    if subtitle = page.subtitle
+      para subtitle, class: "subtitle"
+    end
+    article do
+      section do
+        mount PageContent, page: page
+      end
+    end
   end
 end