Load all files in a data directory: "_all.yml" and other files if present

[jayvdb]

This is a cherry-pick of 6e5dc92 from #67 .

I've tested it, and it works.

I can see some benefit in this, e.g. allowing _all.yml to contain vetted stable entries, or an archive, while new data files are created for new items, which might be migrated to _all.yml at a later date.

A database constraint occurs if the _all.yml and an individual data file contain the same key, which is appropriate. A follow up task can improve that to provide a better contextual message.

[jayvdb]

I'm happy to tidy, add tests, etc.

[thanethomson]

Sounds like a great idea 👍