wrappers: Extend and document the makeNixvim function (#86)

* 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The following information were out of date or incomplete: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

flake.nix

@@ -40,8 +40,14 @@
               packages.docs = pkgs.callPackage (import ./docs.nix) {
                 modules = nixvimModules;
               };
-
-              legacyPackages.makeNixvim = import ./wrappers/standalone.nix pkgs (modules pkgs);
+              legacyPackages = rec {
+                makeNixvimWithModule = import ./wrappers/standalone.nix pkgs modules;
+                makeNixvim = configuration: makeNixvimWithModule { 
+                  module = {
+                    config = configuration;
+                  };
+                };
+              };
             });
     in
     flakeOutput // {