configurable json type

[mathisloge]

Hey,

I'm currently working on a project which depends on the ordering of insertion. nlohmann::json sorts the keys alphabetical.
With nlohmann::ordered_json this behavior could be changed. However inja defines a strong binding to nlohmann::json.

I would like to be able to inject a config file which sets the json type. All files then need to use this type.
but i would dislike to maintain a fork in order to have this behaviour.

If you are open to such a change, I'm happy to provide a PR.