Add --print-config CLI flag (fixes #3127)

[thibaudcolas]

Resolves #3127.

A nice feature of eslint's CLI is the option --print-config that will log to the console the parsed configuration object for a given configuration file, including all the extended rules. It's awesome for debugging configuration files.

This PR is largely inspired by the ESLint implementation. Also recycles its (succinct) docs. Most of the functionality is taken care of by getConfigForFile, so what I added is mostly an integration of that into the CLI, replicating how getConfigForFile gets called in lintSource.

Here are useful commands to use while testing this that should cover the various scenarios:

# The canonical example: point to a file, there's a config with extends nearby, it resolves the full config.
./bin/stylelint.js --print-config lib/__tests__/fixtures/getConfigForFile/a/b/foo.css
# Needs to be given a path.
./bin/stylelint.js --print-config
# Globs are not supported.
./bin/stylelint.js --print-config 'lib/__tests__/fixtures/getConfigForFile/**/foo.css'
# Multiple paths isn't supported either.
./bin/stylelint.js --print-config lib/__tests__/fixtures/getConfigForFile/a/b/foo.css lib/__tests__/fixtures/empty-block.css

Sample output:

$ ./bin/stylelint.js --print-config lib/__tests__/fixtures/getConfigForFile/**/foo.css
{
  "rules": {
    "block-no-empty": [
      true
    ],
    "plugin/warn-about-foo": [
      "always"
    ]
  },
  "plugins": [
    "[...]/stylelint/lib/__tests__/fixtures/plugin-warn-about-foo.js"
  ],
  "pluginFunctions": {}
}

And the errors:

$ ./bin/stylelint.js --print-config 'lib/__tests__/fixtures/getConfigForFile/**/foo.css'
Error: The --print-config option does not support globs.
    at module.exports ([...]/stylelint/lib/printConfig.js:33:7)
    at Promise.resolve.then.then.options ([...]/stylelint/lib/cli.js:478:16)
    at <anonymous>
    at process._tickCallback (internal/process/next_tick.js:188:7)
    at Function.Module.runMain (module.js:667:11)
    at startup (bootstrap_node.js:187:16)
    at bootstrap_node.js:607:3

$ ./bin/stylelint.js --print-config lib/__tests__/fixtures/getConfigForFile/a/b/foo.css lib/__tests__/fixtures/empty-block.css
Error: The --print-config option must be used with exactly one file path.
    at module.exports ([...]/stylelint/lib/printConfig.js:23:7)
    at Promise.resolve.then.then.options ([...]/stylelint/lib/cli.js:478:16)
    at <anonymous>
    at process._tickCallback (internal/process/next_tick.js:188:7)
    at Function.Module.runMain (module.js:667:11)
    at startup (bootstrap_node.js:187:16)
    at bootstrap_node.js:607:3

[gucong3000]

Maybe we need to create a new function to catch errors, not just copy these lines.

[thibaudcolas]

👌 thanks for the suggestion, should be taken care of now. I'm not familiar with the stylelint codebase at all so let me know if there are other parts of the code that could use a refactor or two.

[jeddy3]

LGTM, thanks!

[jeddy3]

• Added: --print-config CLI flag (#3532).