Add deep merge mode

Hi. Thanks for your library, it does the job well. I wanna to add a new feature called "merge mode". Currently if you try to add key to gon, that already exist there it just overrides old value. But with this feature if old value and new value are a hash it will use deep merge to set value. It might be useful in situation when you set complex object in abstract controller, controller callback & action.

To enable it add tiny initializer

```ruby 
#config/initializers/gon.rb
Gon.merge_mode!
```