Add a tool to generate chunks given substack

[dglazkov]

Basically, port https://github.com/dglazkov/ask-my-substack/blob/main/prepare.py here.

[jkomoros]

• [ ] Document how to use the importer. python3 -m import.main --importer substack <DIRECTORY>
• [ ] Figure out how to pass SUBSTACK_URL from the command line (and a general way to pass importer-specific arguments)