WIP: Adding configurable download concurrency

[lucapericlp]

Thanks for the great project! I had to download quite a few chunky videos and thought this might be helpful. The approach is quite naive, I've used temp files to be able to concurrently download (while keeping track of order) then reduce all into the final output file per respective video/audio component. WIP as I'm still making changes but clear to review for the general approach.