config: explore different API designs

[dominicbarnes]

From @dominicbarnes on November 17, 2016 5:11

The more time goes on, the more I want a more robust API around config. Currently, the export of a "static" object is getting more complex as more cases need to be handled during that initial load.

What I think would be best is to create a Config class, which can be constructed from a JSON string or a parsed object. It would exposed methods like Config#entries(overrides) and Config#root() that would perform the relevant normalization. This would allow for a much more easily testable module, in addition to being a lot more robust as an end-user.

There could even be a Config#runner() which creates a trimmed down config that can be passed directly to a mako runner.

Copied from original issue: makojs/config#20