feat(persistence): pre_save option to call before saving (#22)

* feat(persistence): `pre_save` option to call before saving

This commit adds the `pre_save` option, which is a `fun()|nil` that will
be called (if present) before saving the current session. Use cases
include filtering buffers, executing `User` autocmds, etc.

* docs: `pre_save` option

lua/persistence/init.lua

@@ -30,6 +30,10 @@ function M.start()
   vim.api.nvim_create_autocmd("VimLeavePre", {
     group = vim.api.nvim_create_augroup("persistence", { clear = true }),
     callback = function()
+      if Config.options.pre_save then
+        Config.options.pre_save()
+      end
+
       M.save()
     end,
   })