Apologies for putting multiple features in one PR, I was modifying it for my courses while scraping them so they were all added spontaneously.

Happy to split into multiple PRs, though I won't be able to for a while due to IRL stuff.

Features:

• Subtitles are downloaded when the --subtitles flag is passed and only downloaded if they do not exist (Resolves transcript download? #77)
• Lectures containing media files are automatically downloaded. The only one I've seen so far has a mediaType of Presentation and is used to include PDF slides with the lecture video.
• Add --dump-json flag for debugging and for anyone who wants to know what courses were not read etc since the scraping process will mark all videos as read.

Fixes:

• Recursively traverse the course JSON to handle cases where lectures are placed in groups/folders. The group structure will be mirrored in the output directory. Before, any lectures in a group would not be downloaded (which was a problem for some of my courses which made heavy use of this feature)
• Get course name using listing API endpoint which resolves lecture name [[UNTITLED]], content with in its folder keeps cleaned and overrided by newer downloaded video #59
• Retry on error (can happen when doing too many scrapes simultaneously)