Add default values to iterator options

This is an implementation of the default values as described in [`leveldown#iterator([options])`](https://github.com/rvagg/node-leveldown/#leveldowniteratoroptions).

I don't know if the default values on the iterator options hash was left out on purpose, but this should make it easier for users of abstract-leveldown to implement the iterator function since they now don't have to handle default values.