Replace verbose context field with function

[pjones]

Would you be open to these two changes:

1. Use MonadIO instead of IO, and

2. Replace the verbose context field with a function (String -> m ())

These changes would allow integration with existing logging frameworks so migration messages don't go to stdout when logs are going somewhere else.

If these changes sound acceptible I'd be happy to supply a PR.

[ameingast]

Yes, if it's possible to keep the current public API stable. That means no destructive signature changes to existing public functions or data-types - only extensions.

[unclechu]

Implemented #35 in order to solve this.

[unclechu]

@ameingast I closed #35 in favor of #36. Please review.