Add verbose output for updates

[nixjdm]

It would be nice if there was a verbosity level would answer the question, "Why is this artifact being updated?" Having this information available could put a user's curious mind at ease, if they forget what they changed. Or, it could help a developer focus on particular improvements to Lektor's source code.

A complete answer to that question might be hard to come by, but even partial answers might be helpful. For instance, could we output a diff on a changed artifact? I'm not sure which verbosity level would be appropriate here for all of the kinds of answers to the above question; maybe a few. Certainly a diff would slow things down, but I don't think that's very important if you really want that output.