Allow custom handlers for update logs

[mileswwatkins]

It'd be nice to have a way for Pupa to execute user-provided functions on the logs of a scrape or import. That is, after completing an update, take the log that had been produced and sent to stdout, and allow the user to do something with its contents.

The specific case that @jamesturk and I were thinking about would be to have an emailing function or Slackbot, that would send us notifications for particular levels of error or warning. But having a more flexible post-update hook could offer several other options, for Open States or other users.

That said, would this be better accomplished by using a custom real-time logging handler, and then using, say, CloudWatch Logs and custom notifications in a service like that?