kakoune: support for adding plugins

Motivation: There is a thriving plugin ecosystem for Kakoune now,
and it is nice to add these in our Nix configurations. This was modeled
on neovim's plugins.

parinfer-rust is useable both standalone and as a Kakoune plugin,
so the plugin file inherits the same definition as pkgs.

I'll make PRs for other plugins if this gets accepted.
[Here](https://github.com/eraserhd/nixpkgs/tree/kak-ansi)'s a tested
branch for the `kak-ansi` plugin.

doc/package-notes.xml

@@ -325,6 +325,18 @@ packageOverrides = pkgs: {
    elm2nix</link>.
   </para>
  </section>
+ <section xml:id="sec-kakoune">
+  <title>Kakoune</title>
+
+  <para>
+   Kakoune can be built to autoload plugins:
+<programlisting>(kakoune.override {
+  configure = {
+    plugins = with pkgs.kakounePlugins; [ parinfer-rust ];
+  };
+})</programlisting>
+  </para>
+ </section>
  <section xml:id="sec-shell-helpers">
   <title>Interactive shell helpers</title>