Add discount field

[juanpsenn]

A discount field should be added.

[WhyNotHugo]

To ReceiptEntry.

[Alvezgr]

I just needed this enhancement, there is anything I should know?

1. What is the kind of discount needed to be applied here, if it is as a percentage discount or as a pure net discount?
2. discount should be applied to total price or to unit price?

If I'm not wrong, discount need to be validated not to be a negative value, negative value would cause an increment when calculating total_price and also should be less than or equal to unit_price if it is being applied to unit price, a discount greater than the unit price would cause a negative total and I'm not sure what that would be representing,

Thanks.