Implement from_scratch option

[tummychow]

Okay well, I asked for advice in #934, but I accidentally implemented it before getting an answer. Whoops!

Feel free to criticize style, implementation details, naming, tests/lack thereof, etc.

[tummychow]

Ping on this? @sampsyo ?

[jackwilsdon]

That looks good to me, although I'll let @sampsyo have the final call 😂 👍

[sampsyo]

This looks fantastic! Thank you! I only found one tiny typo, which I can fix when I do the merge.

[sampsyo]

Tiny typo: missing one dash in the option name.

[sampsyo]

Sorry for the long delay in reviewing this! Thanks for waiting while it got to the top of my list.

This looks great: it's simple and it matches what users have been asking for. While we're here, I want to briefly take stock of all the ways we have for "clearing out metadata," which have grown steadily:

• The scrub plugin deletes all metadata and writes it back again, which removes tags that beets doesn't know about that show up in files' metadata.
• The zero plugin can selectively remove some metadata fields, or (in "whitelist" mode) only keep certain fields.
• This new feature is sort of like configuring zero to keep only the fields that beets fetches from remote servers. Like zero, it does not remove "unknown" fields.

Some users might want to configure both scrub and this new from_scratch option, or also enable zero to turn off some fields that come from remote sources.

It might be worth documenting this set of choices centrally somewhere, if anyone has a bright idea about where this should live.

[sampsyo]

Merged! ✨ Thank you again for the clean, simple implementation and the readable documentation.

[tummychow]

My pleasure, thank you for merging.

For myself, I actually use all three of scrub, zero, and from_scratch (zero for music that I import as-is, where from_scratch wouldn't apply). I personally think the detailed documentation on scrub/zero is sufficient for me to understand what they do and why I would want them, but my perspective might be skewed after thinking about and implementing this feature.