Added Skipping already downloaded files

[nimafanniasl]

Hey :D
I was trying to use this project and after downloading the dmg file, the script failed to download the chunklist file, I tried to rerun the script but it started downloading the dmg file again.
so I made a simple fix in the code for myself and decided also to make it a pull request :))

[speculatrix]

I think it'd be fine if there's a way to force redownload, so that if the download failed in some way and you needed to discard the partial or corrupted file, you can do that easily.