Syntax highlighting (#691)

* Syntax highlighting mostly... Exists.

* Add dependency to dockerfile

* Handle non-existent languages better

* Make the default a bit nicer

* Improve highlighting. Clean up function

* Add dark theme, add the comment scope to the allowed classes

* update build env

* Address review comments

* Use find_syntax_by_token which produces the desired behavior

* Change flat_map into flatten
(commit cargo.lock)

assets/themes/default/_global.scss

@@ -623,3 +623,28 @@ figure {
     }
   }
 }
+
+//highlighting
+code {
+  .constant{
+      color: $code-constant-color;
+  }
+  .string{
+      color: $code-string-color;
+  }
+  .keyword.type,.keyword.control,.type{
+      color: $code-keyword-color;
+  }
+  .keyword.operator{
+      color: $code-operator-color;
+  }
+  .source{
+      color: $code-source-color;
+  }
+  .comment{
+      color: $code-comment-color;
+  }
+  .function{
+      color:inherit;
+  }
+}