Section prefix for downloads and option for selecting multiple weeks at once

I've added the option to select multiple weeks at once for downloading and a section prefix to the download names. This allows to conveniently create download batches between a single week and all videos. The section prefixes maintain a consistent naming across the downloaded files when downloading individual weeks.

Enter multiple week numbers separated by whitespaces:

    1 - Download Welcome videos
    [...]
    13 - Download them all
    Enter Your Choice: 12 1 8

Or just a single one maintaining compatibility with the previous prompt. The videos will be named as follows:

    01-01-0_WEEK_INTRO.mp4
    08-01-3_WEEK_INTRO.mp4
    08-02-3_wk_orthogonal_bases_01.mp4
    [...]

This branch includes 691c607 which was required for me to get e0c7184 working.