Add changingDeps re. substack/watchify#7

From Issue #7 (now subsumed by this PR):

> It would be great to know, upon an update event is emitted, what files have changed in the latest update.

> This will make it nicer when rebuilds occur to, on the user-side, output this handy information to the log or console and track what has been changed.

This PR seems like the simplest implementation - but just food for thought, if this is a feature you also think watchify should have. Cheers.