add elpa bundle

@senny I always had the `elpa` setup in my personal `local.el` but I think because cabbage is also a great start for people who would like to get into emacs, there should be a `elpa` bundle.

**MELPA** is in my opinion is the largest and most up-to-date repository for Emacs packages. So I think we should add it as a default to the `package-archives`
It's the same condition as above, let's combine them.