Add YAML frontmatter to markdown views with title, author, and URL metadata

Author: dhh

app/views/books/show.md.erb

@@ -1 +1,7 @@
+---
+title: "<%= @book.title %>"
+author: "<%= @book.author %>"
+url: "<%= book_slug_url(@book) %>"
+---
+
 <%= raw @book.markable %>

app/views/leafables/show.md.erb

@@ -1 +1,6 @@
+---
+title: "<%= @leaf.title %>"
+url: "<%= leafable_slug_url(@leaf) %>"
+---
+
 <%= raw @leaf.leafable.markable %>