code: function mixin (target, source, options) {
          options = options || {};
          options.skip = options.skip || [];
          for (let key in source) {
              if (typeof source[key] === 'function' && key.indexOf('_') !== 0 && options.skip.indexOf(key) === -1) {
                  target[key] = source[key].bind(source);
              }
          }
      }
docs: expose all the API methods on the main module using a default instance