New configuration loader

[davidtheclark]

Here's the spec:

• Look for .stylelintrc, stylelint.config.js, and stylelint properties in package.json files.
• Starting from process.cwd(), look for these files in ./ ../ ../../ ../../../ etc., all the way to ~ (user home directory), checking first for stylelint in package.json, second for .stylelintrc, third for stylelint.config.js.
• Stop looking at the first one that is found, and use that one as the config.
• Built-in YAML support for .stylelintrc.
• The config can have an extends property, whose value is a single string or an array of strings. Each string must be a lookup that require() would understand, so either:
  • A npm module name
  • An absolute path
  • A relative path, relative to the config file that is extending
• If config A extends config B, the result is config A's properties merged into config B, with config A taking priority.

The plan right now is to build these features into a standalone module and integrate that module into stylelint.

cc/ #442, #418

[davidtheclark]

This will cause a pretty low-impact breaking change, because the search-and-merge feature of rc will no longer be supported.

[davidtheclark]

Started https://github.com/davidtheclark/load-multiconfig

[davidtheclark]

Mission accomplished in #503

[jeddy3]

🎉 :)

[keithamus]

@davidtheclark awesome work on this! Thanks so much! Is it ready to use (released) yet? I'd love to take it for a spin!

[davidtheclark]

@keithamus You can try the v3 branch. Let us know if you find problems!

[davidtheclark]

@keithamus v3 is now released.

[keithamus]

🎉 I've been working with it a bit - sadly atom-stylelint hasn't yet been updated but my package.json works well. Thanks for all of your efforts @davidtheclark 🍻