Add the ability to cache large downloads

We can enable caching for bash chunks.
The idea would be:

- if `cache=true`
- hash the content of the chunk
- get the folder in which it executed and create a tar archive
- when encountering a same chunk having a similar hash
- restore the archive in the directory before executing the chunk

options with wget can then be used to ignore already downloaded files

cache directory can be in `~/.cache/markdown-runner`

The issue names bash chunks, the cache=true option, and ~/.cache/markdown-runner but no files, tests, or entry points. Start by locating chunk execution and option handling, then determine how hashing, tar archives, and restoration should be tested; done means repeated matching chunks reuse cached downloads without breaking execution.

Written by the indexing model from the issue text.