Function flatten

doc/demo/underscore.js:424–433

(input, shallow, output)

  // Internal implementation of a recursive `flatten` function.
  var flatten = function(input, shallow, output) {
    each(input, function(value) {
      if (_.isArray(value)) {
        shallow ? push.apply(output, value) : flatten(value, shallow, output);
      } else {
        output.push(value);
      }
    });
    return output;
  };

  // Return a completely flattened version of an array.
  _.flatten = function(array, shallow) {