Add highlight.js and highlight.css

`highlight.js` was downloaded from <https://highlightjs.org/download/>
and contains the following languages.
  - Rust
  - TOML
  - JSON
  - plaintext
  - C/C++ (for FFI examples)

`highlight.css` is the same stylesheet used by The Rust Programming
Language book.

Author: ecstatic-morse

static/scripts/highlight.js

@@ -0,0 +1,79 @@
+/* Base16 Atelier Dune Light - Theme */
+/* by Bram de Haan (http://atelierbram.github.io/syntax-highlighting/atelier-schemes/dune) */
+/* Original Base16 color scheme by Chris Kempson (https://github.com/chriskempson/base16) */
+
+/* Atelier-Dune Comment */
+.hljs-comment,
+.hljs-quote {
+  color: #AAA;
+}
+
+/* Atelier-Dune Red */
+.hljs-variable,
+.hljs-template-variable,
+.hljs-attribute,
+.hljs-tag,
+.hljs-name,
+.hljs-regexp,
+.hljs-link,
+.hljs-name,
+.hljs-selector-id,
+.hljs-selector-class {
+  color: #d73737;
+}
+
+/* Atelier-Dune Orange */
+.hljs-number,
+.hljs-meta,
+.hljs-built_in,
+.hljs-builtin-name,
+.hljs-literal,
+.hljs-type,
+.hljs-params {
+  color: #b65611;
+}
+
+/* Atelier-Dune Green */
+.hljs-string,
+.hljs-symbol,
+.hljs-bullet {
+  color: #60ac39;
+}
+
+/* Atelier-Dune Blue */
+.hljs-title,
+.hljs-section {
+  color: #6684e1;
+}
+
+/* Atelier-Dune Purple */
+.hljs-keyword,
+.hljs-selector-tag {
+  color: #b854d4;
+}
+
+.hljs {
+  display: block;
+  overflow-x: auto;
+  background: #f1f1f1;
+  color: #6e6b5e;
+  padding: 0.5em;
+}
+
+.hljs-emphasis {
+  font-style: italic;
+}
+
+.hljs-strong {
+  font-weight: bold;
+}
+
+.hljs-addition {
+  color: #22863a;
+  background-color: #f0fff4;
+}
+
+.hljs-deletion {
+  color: #b31d28;
+  background-color: #ffeef0;
+}