* Add yolov4 example option.
Add yolov4-tiny for benchmark.

* Restyled by astyle

* Restyled by clang-format

* Restyled by astyle

Co-authored-by: Restyled.io <commits@restyled.io>

* Support yolov4-tiny conversion.
Add darknet2ncnn readme.
Add yolov4 example.

* Restyled by clang-format

* Restyled by astyle

* Restyled by clang-format

* Restyled by astyle

Co-authored-by: Restyled.io <commits@restyled.io>

* fuse mish and depthwise convolution

* darknet2ncnn: support enet-coco conversion.

* darknet2ncnn: Add an option, which is enabled by default, to merge all yolo output into one layer. The convert result is compatible with yolov3 example and can be used in benchmark.

* Add darknet to ncnn converter with YOLOv4 support.

* darknet2ncnn: add copyright info, fix clang compatibility

* fix wrong target

* darknet2ncnn: compatible with clang++ 9.0.0