Adding stock and order object to stock threshold notification for better mail customization via hook_mail_alter()

Issue Summary

I am entering this feature request and patch because of what's discussed/requested in #411184: uc_stock notify seller instead of store owner on low stock threashold.

There's a hack proposed there for uc_stock.module which I rather not encourage. I was looking at alternatives way and hook_mail_alter() was the immediate thought, but then I realize I don't have all the information I need for that.

Normally I do not do this, but I really didn't try this patch, it seems like an easy harmless patch so just review for syntax errors, but otherwise, it should be straightforward to be accepted :).