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.

static/scripts/railroad-utils.js

@@ -0,0 +1,6 @@
+import rr from "./railroad.js";
+
+export function railroadDiagram(builder, elementID) {
+    const diagram = builder(rr);
+    diagram.addTo(document.getElementById(elementID));
+}