Added the ability to specify which weeks to download

Added weeks (-w) paramater - you specify which weeks you want to download from, so you don't have to download the whole course.
Example input:

>coursera-dl -u username -p password -w "1,4,5" course

which would download the first, fourth and the fifth week. If "-w" is omitted, it downloads all available weeks.

It assumes that the first number found in week header, is the number of the week and it ignores other numbers (e.g. "Week 1: TCP/IP, 10 rules of something", "Module 10: Example", "10th week: ARC")