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.

plugins/cmp/default.nix

@@ -70,7 +70,7 @@ helpers.neovim-plugin.mkNeovimPlugin {
 
   callSetup = false;
   extraConfig = cfg: {
-    extraConfigLua =
+    plugins.cmp.luaConfig.content =
       ''
         local cmp = require('cmp')
         cmp.setup(${helpers.toLuaObject cfg.settings})