optimize and fix some erros

optimize

• render html files with absolute path for images
• optimize file and folder naming

fix

• fix error when the file name contains special characters in windows
• fix subtitle download

Fix edx-dl to work and major tweaks

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