nixos-option: Complete custom modules as well
nixos-option: Complete custom modules as well

Until now `_nixos-option` simply includes `module-list.nix` from
`<nixpkgs>` which contains all upstreamed modules. This won't work with
custom modules that are used in the current `<nixos-config>`. By
evaluating `<nixos-config>` (if available) all remaining modules will be
taken into account as well.

spwhitt/nix-zsh-completions