transfers/pipeline: config to disable ACH file merging

[adamdecaf]

What were you trying to do? ACH file merging is pretty strict right now and enabled by default. There are use-cases that want to perform their own merging and thus PayGate should offer to disable merging and instead upload each Transfer in a unique file.

What did you expect to see? A configuration option to disable merging until we can offer more flexible merging options.