Support ffmpeg and video intro, outro and transitions

[romanrsr]

It would be nice to be able to add support for ffmpeg, since it supports the use of GPUs. For example: ffmpeg_command.extend(['-filter_complex', 'concat=n={}'.format(len(video_files)), '-c:v', 'h264_nvenc', output_file])

And also the possibility of adding a path with videos of intros, outros and transitions.

Thanks for the work you do!