Add Tab's railroad.js to the blog

- Include railroad.js from http://tabatkins.github.io/railroad-diagrams/
- Include railroad.css from the same, and tweak it to match my style a bit
  better
- Add a railroad-utils.js file that declares a railroadDiagram function. It
  takes a builder function and an element ID, calls the builder and adds the
  resulting diagram to the element with the given ID.
- Add a railroad_diagram shortcode that wraps that all up so you can write a
  diagram inline with a post.
- Add script includes if the railroad_diagram shortcode is used. The shortcut
  sets a variable in the page's .Scratch that is looked up in the head partial
  template to determine whether to include these scripts.

layouts/shortcodes/railroad_diagram.html

@@ -0,0 +1,9 @@
+{{ $id := .Get "id" }}
+{{ .Page.Scratch.Set "includes_railroad_diagram" true }}
+<figure class="railroad-diagram" {{ if $id }}id="{{ $id }}"{{ end }}></figure>
+<script defer type="module">
+    import { railroadDiagram } from {{ `/scripts/railroad-utils.js` | relURL }};
+    railroadDiagram(rr => {
+        {{ .Inner | safeJS }}
+    }, "{{ $id }}");
+</script>