OpenCV DNN GPU support

First of all I really appreciate this repo and all the contributors.

I want use OpenCV DNN module for inference.

Here are the details of installation:

1. Latest **opencv4nodejs**
1. OpenCV version: 4.3.0
1. Build Additional flags: `-DWITH_CUDA=ON -DWITH_CUDNN=ON -DOPENCV_DNN_CUDA=ON`

Build has been successful. Currently I can use *DNN* module only on CPU.

In openCV documentation I have found [setPreferableBackend](https://docs.opencv.org/4.3.0/db/d30/classcv_1_1dnn_1_1Net.html#a7f767df11386d39374db49cd8df8f59e) and [setPreferableTarget](https://docs.opencv.org/4.3.0/db/d30/classcv_1_1dnn_1_1Net.html#a9dddbefbc7f3defbe3eeb5dc3d3483f4)
that is used to set **CUDA** Backend. But I could not have found similar API in this repo.

I am asking this as a **feature request** or any workaround is welcome.

*OpenCV version* : 4.3.0

*With OpenCV-contrib?* (extra modules): yes

*OS*: Ubuntu 18.04