Invoices: Ability to add a custom column

[Douggie66]

Please add the ability to add a custom column to invoices. I would like to be able to display a discount column so that customers can see what discount I have applied to certain items.

"Add Columns to Invoice"

[Shazzy]

I would also like to add columns, I would like to be able to add a Date Column to be able to add daily hours and explanation of work on the one invoice.