Use cosmiconfig for reading configuration

[thasmo]

cosmiconfig from David Clark, which is also used in stylelint is a nifty tool to find and read configuration files.

Short excerpt of what it's doing:

Find and load a configuration object from
- a package.json property (anywhere down the file tree)
- a JSON or YAML "rc file" (anywhere down the file tree)
- a .config.js CommonJS module (anywhere down the file tree)
- a CLI --config argument

[titanism]

@adrienverge @benedfit @ForbesLindesay @TimothyGu can we get this added? If you'd like us to help maintain this package, please add titanism to GitHub admins for pugjs org and pug-lint on npm ownership if possible.