Fix:
* fix .unescape error in Python 3.9 and above
* fix course outline pasing error
* fix unit test error when cross-platform test
* fix youtube video download when --prefer-cdn-videos option already provided
* fix github actions workflow to support coveralls
* fix syntax error
Features:
* add --browser-cookie argument to specify automatically extract CAUTH value from the browser cookie.
* add support for login using browser, and --headless argument can be used to contronl.
* add downloading progress bar
* add thread-based parallelism for downloading
* add support for unit as html file
Refactor:
* rename project
* update python version of support
* move test files
* optimize code