Arbitrary level backends

[nolanlawson]

It looks like we could easily support something like

--level-backend riakdown --level-prefix riak://localhost:8087/

With the requirement that you npm install it yourself. It looks like that would cover riakdown, redisdown, and sqldown. And it would save me from writing all these separate options that do the same thing:

• https://github.com/pouchdb/pouchdb-server/tree/redis
• https://github.com/pouchdb/pouchdb-server/tree/sqldown
• https://github.com/pouchdb/pouchdb-server/tree/riak

[nolanlawson]

a083a9b