Support ACH stop orders

[matin]

It's possible to cancel an ACH transaction before it has cleared. This can be useful if the transaction was submitted accidentally, or if fraud has been detected.

See also #2 and #3

[matin]

There is definitely value in supporting stop orders, but it will complicate the API. If anything, it will require effort to think about the spec and determine when the stop order can and can't be issued since it's not guaranteed.

The value here, at least for now, is not worth the effort of providing the functionality.