feat: allow to disable execution_message

Squashed changes from https://github.com/Pocco81/auto-save.nvim/pull/50
Got too hairy for a GH noob like me to fix properly.

lua/auto-save/config.lua

@@ -2,6 +2,7 @@ Config = {
 	opts = {
 		enabled = true, -- start auto-save when the plugin is loaded (i.e. when your package manager loads it)
 		execution_message = {
+			enabled = true,
 			message = function() -- message to print on save
 				return ("AutoSave: saved at " .. vim.fn.strftime("%H:%M:%S"))
 			end,