Adds support for code references on code pages

Example:

set: 2
code: 200
title: OK
references:
    "Rails HTTP Status Symbol": “:ok”

templates/code.jade

@@ -9,4 +9,8 @@ block contents
       h2 !{groups.codes[set].title}
       h1 #[span !{code}] !{title}
       != contents
+      if references
+        h1 #[span !{code}] Code References
+        each value, key in references
+          p !{key} !{value}
       a(href='/') ← Return to httpstatus.es