Add face detection

[i-Still-Believe]

Models are here!
https://drive.google.com/open?id=1muCvAjATO7Mi7YbtCAbLJiNgGByDHDwH
https://drive.google.com/open?id=1rGYQvIcuxLOQdRYYcUYZNvsHdQlYcqco
https://drive.google.com/open?id=1lYLQU3GFR33R96D0-T-2AThIw93wF54L
https://drive.google.com/open?id=1GkShcm9Zp_vAohyrLt2ITdm3ftHQ4gUc

You can use --face to use face detection. Default is Not.
You can use --hand to use hand detection. Default is Not.

[Fang-Haoshu]

Thanks jiasong! Nice works.

Here are some modifications before merging.

1. I found that there are some *.pyc files, could you please remove them and add *.pyc to the .gitignore
2. The flag should be --face. And we don't need a write_face. You can append the face keypoints after the original keypoints, that means, we have 51+17 keypoints now. We do not need to render the face independently. You can move that part into vis_frame and vis_frame_fast
3. we can move all the centerface model files to google drive
4. we can rename centerface to face
5. There are some useless files under centerface/utils. Please check them and remove all useless files and functions

[ZhengLeon]

Hi@ @7-GUO-7 ,
The face detection models you shared are unavailable now, could you please
upload again?