hot fix for issue #123
Set execute permission on script

Having the script executable makes command line completion way more
comfortable.
Ignore temporary files from Vim
Add semi-interactive password query

A password supplied as an argument is likely to end up in the command
line history. Just querying the password interactively on demand allows
to conveniently supply everything else as command line arguments.
Add support for selecting multiple weeks at once

This includes a simple handling for parse errors.
Prefix downloads with courseware section number

This maintains a consistent naming and ordering for downloaded videos
even in case the individual weeks are downloaded separately.