fix(aggregate): add chainable .option() helper for setting arbitrary …

…options

Fix #5829

lib/aggregate.js

@@ -542,13 +542,31 @@ Aggregate.prototype.explain = function(callback) {
  */
 
 Aggregate.prototype.allowDiskUse = function(value) {
-  if (!this.options) {
-    this.options = {};
-  }
   this.options.allowDiskUse = value;
   return this;
 };
 
+/**
+ * Lets you set arbitrary options, for middleware or plugins.
+ *
+ * ####Example:
+ *
+ *     var agg = Model.aggregate(..).option({ allowDiskUse: true }); // Set the `allowDiskUse` option
+ *     agg.options; // `{ allowDiskUse: true }`
+ *
+ * @param {Object} value keys to merge into current options
+ * @see mongodb http://docs.mongodb.org/manual/reference/command/aggregate/
+ * @return {Aggregate} this
+ * @api public
+ */
+
+Aggregate.prototype.option = function(value) {
+  for (var key in Object.keys(value)) {
+    this.options[key] = value[key];
+  }
+  return this;
+};
+
 /**
  * Sets the cursor option option for the aggregation query (ignored for < 2.6.0).
  * Note the different syntax below: .exec() returns a cursor object, and no callback