Billing: Optional items in offers

[netzmacht]

We often have to add some optional items or alternatives to our offers which shouldn't get included in the total price. It would be great if GO supports it.

• Allow to mark an item as optional in the offer book
• Optional items should not be included in the total sum
• Optional offer should be highlighted in the documents/pdf (e.g. by displaying item total in braces)
• When converting an offer to a bill it should be possible to decide which optional item were booked and has to be paid.