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.

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")