Allow external db instance

[Azarattum]

Is your feature request related to a problem? Please describe.
Currently y-leveldb takes a level constructor but not a db instance. Therefore there's currently no way to pass an initialized database to it.

Describe the solution you'd like
Add {db: ...} in options to pass a database.

Describe alternatives you've considered
If this is technically not possible (because of the encoders, etc). It would be nice if the external database could be exposed. So, it's possible to query it/apply plugins directly.

Additional context
In general I'm looking for a way to do full-text searches in my yjs documents which are stored in leveldb. Therefore I want to use an indexing plugin like search-index. Not sure if that's the right approach though. If there are better ways to index yjs docs, please let me know.

[beorn]

Agreed - see also #17
Instead of passing a level creator and options you can just pass a level instance (abstract-level compatible) that's been pre-created with the right options.