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The following information were out of date or incomplete: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

wrappers/standalone.nix

@@ -1,4 +1,4 @@
-pkgs: modules: configuration:
+default_pkgs: modules: {pkgs ? default_pkgs, module}:
 
 let
 
@@ -7,7 +7,7 @@ let
   wrap = { wrapRc = true; };
 
   eval = lib.evalModules {
-    modules = modules ++ [ { config = configuration; } wrap ];
+    modules = (modules pkgs) ++ [ module wrap ];
   };
 
 in eval.config.finalPackage