incorporating audio into output videos

[leios]

Right now, VideoIO.jl does not support Audio encoding, so it is probably best to create a video the ol' fashioned way with ffmpeg by either creating a video from images, ie:

ffmpeg -r 30 -i ../check%05d.png -i audio.mp4 -c:v libx264 -pix_fmt yuv420p output.mp4

or using Fable.jl to make the video and then

ffmpeg -r 30 -i video.mp4 -i audio.mp4 -c:v libx264 -pix_fmt yuv420p output.mp4

Related issue https://github.com/JuliaIO/VideoIO.jl/issues/7