Add persistence (write)

[philippgille]

This is the first of at least 3 PRs for adding persistence to chromem-go.

1. Write data when adding collections/documents <-- we're here
2. Read data from files when creating new DB
3. Delete data on collection deletion or DB reset

Apart from these three mandatory features, future additions are envisioned: Encryption, choice of encoding (e.g. JSON in addition to gob), choice of storage (e.g. via https://github.com/philippgille/gokv), compression via gzip, and many more