Feature request (minor): clean up after download

[merberich]

Running the script leaves several JSON and PKL files in the top level directory, as well as a 'resource' directory at the first level (presumably for generating PDFs and/or HTML). I usually clean these with an automated script after running dl_coursera, but perhaps it could be a built-in feature (via some temp directory that gets destroyed easily?).

[FLZ101]

The resource directory contains files used by HTML materials downloaded. Others can be deleted.

Maybe converting HTMLs to PDFs when downloading is a good feature. In the next release, nothing except course materials downloaded should exist in the destination directory specified.