Correctly handle funds type fields

[barszcz]

Bai2 Version: 0.1.0

What were you trying to do?

What did you expect to see?

What did you see?

How can we reproduce the problem?

This is a follow-on from #52. Right now the library doesn't properly handle funds types (see page 30 of [the spec]), instead coalescing all fields from funds type onward in a given record into a slice of strings.

Ideally we would have some kind of rich type here that captures all the detail in this. For a first pass just grabbing a slice of strings might be fine, as long as we can distinguish them from the fields that come after them in 16 (transaction detail) records.

Also I just discovered that 03 (account identifier) records can have more than one of these. It can have the sequence of (type code, amount, item count, funds type) once per type code, an arbitrary number of times. I'll open another issue to address that as well.