Format headers more idiomatically

Related to https://github.com/milkypostman/melpa/pull/1016

* Move `Package-Requires` next to related headers
* Use `;;;` only for section markers
* Add `Code:` section marker

Separately, I saw the `Local-Variables` section and wanted to let you know that you can enable `rainbow-mode` more easily:

```lisp
;; Local Variables:
;; rainbow-mode: t
;; End:
```