neutrino: add a prototype of a lua plugin interface

this is just for preliminary tests, not yet really usable
for anything useful

src/plugin.h

@@ -39,7 +39,8 @@ typedef enum plugin_type
 	PLUGIN_TYPE_DISABLED = 0,
 	PLUGIN_TYPE_GAME     = 1,
 	PLUGIN_TYPE_TOOL     = 2,
-	PLUGIN_TYPE_SCRIPT   = 3
+	PLUGIN_TYPE_SCRIPT   = 3,
+	PLUGIN_TYPE_LUA      = 4
 }
 plugin_type_t;