(#2415) - add LEVEL_PREFIX option for riakdown etc.

Forgot to add this for riakdown etc.  Gonna open this up for testing, though, so we can validate that it actually works for *DOWN authors.
a couple options to make this easier

- use self.constructor, so PouchDB.replicate always uses new PouchDB, but if you have ModifiedConstructor then you can use ModifiedConstructor.replicate
- finally depreciate PouchDB.replicate in favor of db.replicate.to/from
- require people to pass in a db object if they care how its made