* wrappers: Allow to customize the nixpkgs used for nixvim
This allows to pass overlays and other such modifications of nixpkgs.
* wrappers: Allow to pass a custom module to nixvim
This is useful to be able to take full advantage of the Nix module
system, with `imports` and `options`.
* README: Update the documentation on the standalone usage
The following information were out of date or incomplete:
- The `build` function has be changed to the `makeNixvim` function.
- `makeNixvimWithModule` has been introduced in order to allow more
customization.
- Added a full example using nixvim in a standalone flake
