Handle async data in registration function

[SomeKittens]

Registration functions may need to run async things before the registration is ready to go.

Example:

gustav.transformer('isSpam', iO => {
  // How do we return iO?
  natural.BayesClassifier.load('classifier.json', null, function(err, classifier) {
    // Subscribe to flow and run things past the classifier
  });
});

[SomeKittens]

One option is to return a new observable stream, and flatMap or somesuch so we can do some async task inside and emit our own Observable (which then emits data).