in_use parameter added to the coupon loop

This PR adds an optional `in_use` parameter to the coupon loop. if `in_use = true`, the coupons currently used in the checkout process are returned. If `in_use = false`, only coupon not used in the checkout process are returned.

The loop returns a new variable, `$DISCOUNT_AMOUNT`, which is the amount subtracted from the cart total by the coupon.