Makr config readable by emacs-overlay

[Chris Warburton]

The "emacs overlay" for nixpkgs can read use-package declarations from a config
file, so we should make our config compatible with it. This lets us do
declarative package management, with the advantages of use-package (i.e. that
the definitions are Emacs-readable, and allow configuration, hooks, etc.) *and*
the advantages of Nix (fixed inputs, reproducibility, arbitrary dependencies,
etc.)

See https://github.com/nix-community/emacs-overlay