pipeline: support balancing files before upload

[adamdecaf]

What were you trying to do?

Certain FI's require that ACH files are balanced (adding an "OFFSET" record that adds a credit/debit record so the batch sums to zero) before uploading. The ACH library supports this, but paygate has no support for doing this prior to upload.

What did you expect to see?

A configuration option (per routing number) to enable this prior to upload.

We can use the ACH library: https://godoc.org/github.com/moov-io/ach#Batch.WithOffset