Updated data storage structure?

[boldandbrad]

Adopt a new data structure that reduces duplicated data and simplifies how meeple-cli handles data updates.

Tracked boardgames and expansions are stored in one yaml file at ~/.meeple/collections.yaml. BGG-IDs are handled at the root, with the collections they belong to nested beneath.

- 12345
    collections:

And store data for all board games in one place. That way if board games move between collections... but then we can't see accurate history of adds and drops from collections. idk.

[boldandbrad]

This is being sidelined for now since storing data in this form would not allow us to complete #21