Repeat only incomplete downloads with wget when re-running download.sh

[johann-petrak]

If download.sh gets interrupted and re-run, all files will get downloaded again.
Adding these wget options will automatically only download the still incomplete files and only the part that is missing:
"-N -c".
Other options which may be useful to add could be "--progress=bar:force:noscroll --tries=1 --timeout=5 -nv"

[gone-phishing]

I've added the -N and -c option to wget in the latest commit. Thanks for pointing that out :)