Add formatter property in configuration

[ybiquitous]

What is the problem you're trying to solve?

If a formatter in a configuration (e.g. stylelint.config.js) is supported, we can simplify stylelint CLI arguments or specify a formatter via an environment variable. For example:

Before:

stylelint a.css -f json
stylelint b.css -f json
# ...

After:

stylelint a.css
stylelint b.css
# ...

// .stylelintrc.json
{
  "formatter": "json"
}

// .stylelintrc.js
export default {
  formatter: process.env['STYLELINT_FORMATTER']
};

What solution would you like to see?

• Add a formatter property in a configuration

// stylelint.config.js
import myCustomFormatter from 'my-custom-formatter';

export default {
  // A built-in formatter name
  formatter: 'string', // | 'compact' | 'json' | ...

  // Or a path to a custom formatter module
  formatter: './path/to/my-custom-formatter.js',

  // Or a custom formatter module
  formatter: myCustomFormatter,

  rules: { /* ... */ },
};

• It's possible to override the property by CLI arguments: --formatter or --custom-formatter

Background: #7617 (comment)