added support for two streams

In some cases, a transform is actually split over two streams, not just one. This PR adds support for optionally passing in two streams, separating `writable` and `readable`.  

(having to `duplexer(writable, readable)` before calling `wtr` is adding a lot of complexity, because those streams are [already duplexed](https://github.com/jbenet/node-pipe-segment-codec/blob/master/index.js), etc.)