lib/neovim-plugin: Add lua configuration scoped to the plugin

This commit adds a `plugins.<name>.luaConfig` section controlling the
plugin specific configuration.

The section contains the internal `init` option, containing the plugin's
initialization code.

It also contains the public `pre` and `post` options, that allow to add
code before & after the `init` section

Finally, it contains the `final` option, being the concatenation of the
three previous options.

lib/utils.nix

@@ -77,6 +77,12 @@ rec {
     in
     lib.concatStrings (map processWord words);
 
+  /**
+    Those helpers control the lua sections split in `pre, content, post`
+  */
+  mkBeforeSection = lib.mkOrder 300;
+  mkAfterSection = lib.mkOrder 2000;
+
   /**
     Capitalize a string by making the first character uppercase.