Make it easier to store config outside of the project, to smoothen the upgrade process

[alice-mm]

Currently, configuration customisations have to be done directly in the project’s own config/bandcamp.sh file. This means that it may be overridden (or create conflicts) when trying to fast-forward to the latest version of the project.

→ Try to allow to use an optional config file in the user’s home directory. This would either be loaded alongside the config/bandcamp.sh file or replace it altogether.
Perhaps we should use readonly less frequently to allow overrides in that personal file. Not sure how to organize this stuff yet.