UnderscoreJS-inspired `merge` taking advantage of ES5 where possible

merge.js

/**
 * UnderscoreJS inspired `merge`
 * UnderscoreJS calls it `extend` (overloaded in JS); `merge` feels more intention revealing
 * This is naive as it always overwrites if the target property exists.
 * 
 * @see    https://github.com/documentcloud/underscore/blob/master/underscore.js#L727
 * @param  {Object} target
 * @return {Object} target
 */
function merge(target) {
  [].slice.call(arguments, 1).forEach(function(source) {
    Object.keys(source).forEach(function(key){
      target[key] = source[key];
    });
  });
  
  return target;
}