“Sell” and “Buy” variables#613

I think it would be cool to have variables like {sell:} and {buy:} to go with the addition of {modifyinv:}. We also have the {modifybal:} variable so I think we’ll and buy variables would fit in.

{buy} = {modifyinv:[item]|+y} {modifybal: -x}
{sell} = {modifyinv:[item]|-y} {modifybal: +x}