Support a ConfigLogger interface

[cbeck88]

One thing that I did experiment with recently was having 3 layers in figment - clap, toml config file, and defaults. In the web app I was able to present a page that showed these layers of values as keys and values. This approach seems useful in the case of writing a web app that is a product (as opposed to where the webapp is the development activity of an organization). The page becomes a sort of answer to two questions simultaneously "how is this configured", "why is this configured with that value?". Does this seem like something that could be handled by conf (with changes obv.)?

I definitely like this idea -- I think it should be in scope to either always generate some kind of report like this, or provide some kind of hook or customization point that can be used to do it. I'm going to make an issue to follow up on this specifically

Originally posted by @cbeck88 in #1 (comment)