feat: allow extends to load configuration from a module

[bcoe]

Rather than adding more configuration for the extends feature, I've opted to simplify our logic somewhat:

it's now always assumed that either:

• extends, regardless of whether it comes in through pkgConf or config, points to the absolute path of a configuration file -- this approach might not solve 100% of use-cases, but I like how much it simplifies the problem.
• extends may now also represent the name of a module to require (this assumes that the module exports configuration in its index, see https://www.npmjs.com/package/yargs-test-extends) -- I think this feature allows users to build something resembling babel-presets on top of yargs!

BREAKING CHANGE: extends functionality now always loads the JSON provided, rather than reading from a specific key

@rcoy-v great work on the extends feature; I'm really excited to pull this into nyc.