pl.write() never writes if opts are not set

I might be missing something, but shouldn't we have sensible defaults for `windowSize` and/or `windowTime` [here](https://github.com/dominictarr/pull-level/blob/master/index.js#L68)?

I had an issue with data not been written to leveldb and I think it is because window.recent() buffers forever when both `size` and `time` are nulli/undefined.
