Backup everything from Trello:

* boards, open and closed, as json files
* lists, open and archived, as json files
* cards, open and archived, as json files
* attachments, downloaded as raw files

The script also creates a folder tree structure corresponding to
the way data is organized. This is to make it more convenient to
navigate locally between folders, as it mimics the flow you have
when using the web and mobile apps.