Release 0.1.0

[ejwick]

This PR, tentatively named Release 0.1.0, includes several code improvements that are part of a wider scope of changes that I want to make on my fork. Here is a summary:

• Tidied and refactored the code, introducing the spotifybackup package
• Added pyproject.toml to make the script pip-installable (to prepare for a PyPI release?)
• Made extensive use of Python's standard library, idioms and best practices to reduse repetition and improve performance

I've raised this because it doesn't change the CLI, so I think it can be safely merged without much confusion to existing users (who probably just saved the script once anyway).

On my fork, I've tagged this as v0.1.0 (@caseychu could use the same tag); this major version represents non-breaking changes to the original script. The first v1 release will represent a departure from the original interface.

[ejwick]

Probably should've raised from my tag v0.1.0 rather than master, but it seems I can't change that or raise a new PR.