configurable json type

[mathisloge]

This is the implementation of https://github.com/pantor/inja/issues/213

Currently there is one breaking thing in the code base.

If you choose nlohmann::ordered_json the pointer https://github.com/pantor/inja/blob/2d515078c647457436556763aca8d4bf7d11d5e8/include/inja/renderer.hpp#L39 will get invalidated each time a new key was added.

There are two possible ways to fix that, either update the pointer each time json_additional_data is modified or decouple current_loop_data from json_additional_data.

[eadlam]

@mathisloge / @pantor, I could really use this feature. How hard do you think this is to finish?