Hooks for post-processing

I wanted this software to create a .pls of the downloaded mp4 files for the course, but decided on the more general "run this script after downloading" approach, putting the pls-creation code in a script and telling coursera-dl to run it in the correct directory after finishing the downloads.

I'm not a python coder, so I recommend giving it a good eye over—the changes are small, but perhaps dangerous (is os.system safe?).