fix: make frontmatter layout optional (#38)

Make the frontmatter layout optional, falling back to the configured
layout

Author: paradox460

lib/tableau/extensions/post_extension/posts/post.ex

@@ -9,7 +9,7 @@ defmodule Tableau.PostExtension.Posts.Post do
     attrs
     |> Map.put(:body, body)
     |> Map.put(:file, filename)
-    |> Map.put(:layout, Module.concat([attrs.layout || post_config.layout]))
+    |> Map.put(:layout, Module.concat([attrs[:layout] || post_config.layout]))
     |> Map.put_new_lazy(:title, fn ->
       with {:ok, document} <- Floki.parse_fragment(body),
            [hd | _] <- Floki.find(document, "h1") do