Darknet support

Thanks a lot for this, guys, it works great for quick tests of various models.

I wonder if you have considered extending the backend to also support Darknet. I know there are various conversion tools to Keras or TensorFlow, but I'd rather drop the native model files and use the `darknet` library instead of having to go through additional steps.

If you don't mind me doing it, I can take an initial look at how such an extension could work.

Congratulations anyway for your work!