Support for custom values on build/create methods.

Hello!

This is a feature from factory_girl that I missed in Monky.

`create` and `build` methods with custom optionals values for a specific instance.

Example:

```js
mongo.create('User' {city: "California", email: "custom@email.com"}, function(err, user){
  // user ==> saved mongoose user model with custom values
})
```
Hope can be useful.