feat: make possible to define official formatter as string

[ricardogobbosouza]

This PR contains a:

• bugfix
• new feature
• code refactor
• test update
• typo fix
• metadata update

Motivation / Use-Case

The formatter option only accepted function, with this feature we can also set official formatters as string

const StylelintPlugin = require('stylelint-webpack-plugin');

module.exports = {
  // ...
  plugins: [
    new StylelintPlugin({
      formatter: 'json' // See https://stylelint.io/user-guide/node-api#formatter
    )
  ],
  // ...
};

Additional Info

Resolve #201

[codecov-io]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (75a9ef4) to head (586e0ac).
Report is 136 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #202   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            6         6           
  Lines          112       121    +9     
  Branches        24        27    +3     
=========================================
+ Hits           112       121    +9     

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