Allow cron query to be altered by other modules

We are working on a site where we would like to change the logic that used when deciding whether a product is available, and therefore whether emails should be sent out. Currently there's no clean way of doing this without modifying the module directly. If the query that is used inside commerce_stock_notifications_cron was tagged (https://www.drupal.org/node/310077) then we could apply our changes cleanly with a separate module.

Patch attached.