Added parameter to create playlists after downloading a class section

Hey guys, I saw some discussion on how to generate a playlist after processing in #61, however since I'm running on windows (my bad), I thought it could help if those playlists were generated at runtime, since m3u files are so simple to be built. So I added a parameter to help me with the task and it generates a playlist with all the mp4 files (guess this is coursera standard) and after finishing to download the section, it builds the m3u file. I hope you guys find it useful :D