Allow returning a stream

I've made some experiments with [rollup-stream](https://github.com/Permutatrix/rollup-stream) and rollem to try to glue them together and it seems it would be much easier if we simply added an option like `options.stream`. Instead of simply returning a list of destination, it could be a stream. This would allow us to use other Gulp modules more easily.

Start by tracing rollem's current return path, which returns a list of destinations, and compare it with the linked rollup-stream experiment. Determine how an options.stream mode should expose the result for Gulp modules, then verify that the existing list behavior remains unchanged and that the stream mode works.

Written by the indexing model from the issue text.