Add options to configure the RRR specific tva code

Since we want the RRR entries to be written within a single book entry,
we need to group all discounts that have different tvas configured.
We've chosen to create a disctinct tva in the treasury and to use its
compte cg and code to group our discounts.
Add the discount lines in the sage export

Discount lines are treated independently from the other invoice lines.
They are grouped into a single book entry under a tva specific to the
RRR account.