support for pro-forma 13-column Account fees

[braverock]

The NFA 13-column report is described here:

https://www.nfa.futures.org/NFA-compliance/publication-library/disclosure-document-guide.pdf

blotter currently supports most of the features of the 13-column report.

It would be desirable to add pro-forma fees to a blotter account on a prospective basis after the fact, so that the effects of different fee structures could be examined.