Add logging machinery

[hbagdi]

As of today, there is no proper logging machinery in decK. There exists a --verbose flag in decK which is used only in a couple of places in an ad-hoc manner. There have been instances where logging would have been helpful but I put that off for later and never got around to it.

This issue is the result of this discussion: https://github.com/Kong/deck/pull/226#discussion_r535423549