feat: add cache

[ruyadorno]

I was playing with this idea of having a --cache option built into branch-diff with the intent of helping avoid hitting the GitHub API rate limits as frequently when working with long-lived release lines.

I'm not so sure about it anymore since I believe it might be simpler to just storing branch-diff results to a local file might satisfy 90% of the use cases.

Opening this as a draft so that we can discuss.