Dynamic configuration attributes

This pull request is a first stab at what's been (somewhat) briefly discussed in #45, and is not a final solution as of right now, but more of a playground to see how it works.

It'll introduce quite some breaking changes, so I'd like to think it through thoroughly and make sure that there's tests for possible corner cases, etc, etc. But it feels like a pretty maintainable solution compared to what's currently in `master`. I do like me some comments in configuration files, but I can definitely live without them.

Let's see how this goes.
Perhaps, but then I'd have to resort to do something like `Chef::Resource.send(:include, Kafka::Configuration)` (so what you wrote above), which I'm personally not a big fan of. By "bounce" I assume you mean that the `server.properties.erb` template can be removed from the cookbook?