prevent the mutability of config (#34)

Co-authored-by: Anton Khmyrov <akhmyrov@yandex-team.ru>

Author: vivcogit

index.js

@@ -50,7 +50,7 @@ module.exports = function(source) {
  }
  }
 
- var svgo = new Svgo(config);
+ var svgo = new Svgo({ ...config });
  svgo.optimize(source, { path: this.resourcePath })
  .then(function(result) {
  callback(null, result.data);