code: function mergeHelpers(engine, options) {
    if (!options || typeof options !== 'object') {
      throw new TypeError('expected an object');
    }
  
    var opts = extend({}, options);
    var helpers = merge({}, engine.helpers.cache, opts.helpers);
    var keys = Object.keys(helpers);
    for (var i = 0; i < keys.length; i++) {
      var key = keys[i];
      engine.asyncHelpers.set(key, helpers[key]);
    }
  
    opts.helpers = engine.asyncHelpers.get({wrap: opts.async});
    return opts;
  }
docs: Merge the local engine helpers with the options helpers.
  
  @param  {Object} `options` Options passed into `render` or `compile`
  @return {Object} Options object with merged helpers