Allow get_availability to accept quantity parameter.

In some cases where a product must be sold in increments (extensions such as Bundles may enforce a certain quantity on a purchase, for example), it might help to have availability information taking a min quantity into account.

For example, a product sold in a bundle in fixed increments of 2 should appear as out of stock / on backorder *inside the bundle* if there is only 1 item left. 

Perhaps the ```$this->get_total_stock() >= $quantity``` conditional should go next to ```$this->get_total_stock() > get_option( 'woocommerce_notify_no_stock_amount' )``` with a ```&&```?