Store board configuration

I think it would be a nice thing if we could be able to store multiple boards to a specific file location instead of at $HOME

For instance, we could have shared taskbooks accross git repositories using a single JSON file

Start by locating the CLI entry point and the code that currently stores boards under $HOME. Determine how a user-specified file location could support multiple boards shared across repositories, then verify that boards can be stored and loaded from a single JSON file without relying on $HOME.

Written by the indexing model from the issue text.