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

Merging #202 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #202   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           6      6           
  Lines         112    121    +9     
  Branches       24     27    +3     
=====================================
+ Hits          112    121    +9

Impacted Files	Coverage Δ
src/getOptions.js	100% <100%> (ø)	⬆️

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