Add a new i18n module

@omarreiss @andizer @tacoverdo could you please review this? If it works, I'll add it to all the other plugins too. Feedback on both this pull as well as the [module](https://github.com/Yoast/i18n-module) underneath it is more than welcome.
Suggestion: move this up to be within the `if ( in_array( $pagenow, array( 'admin.php' ) ) ) {}` clause as it's only used on the WPSEO admin pages. This would prevent an unnecessary extra include on non-wpseo admin pages.