(#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.
This was the part I was most unsatisfied with. In order to get `PouchDB.replicate` to work, I needed to pass in the constructor as an option, since otherwise it just calls `new PouchDB` no matter what the constructor is. I can't think of any better way to do this.