add .sub option for passing in your own sub db

this is intended to address https://github.com/rvagg/node-level-ttl/issues/15

the idea is simply that you can pass your own levelup-ish instance in

I tested this with level-session and it fixes the issues with level-sublevel v6

another idea for the option name I had was `ttldb` rather than `sub`. LMK if you like that more and I can update it