Adding Level adapter support for Sublevel and Multilevel

I needed support for both these for a current project. Fairly straightforward implementation using `Net.connect()`.

Summary of changes:

- Update local module requires to support `LevelUp` and `Multilevel` as well as `Level`
- Update adapter init to branch logic for Multilevel vs. plain Level
- Update config to support a sublevel name, and handle in init (will attempt a local require for level-sublevel, and throw an error if it doesn't exist).
- Update config to disable / modify `keyPrefix` (to avoid duplicate prefixes when using sublevels)
- Add connect/disconnect methods which I really uncertain of. Docs indicate they are just for getting connection feedback, but the mongo adapter seems to use them for acquiring the db connection itself. I followed the docs.