Add #merge method

I wanted a way to merge two streams, similarly to `concat` but without having to wait for the first one to end to start consuming the second one. This would be useful when you don't care about the order values are pulled from each stream into the merged stream (although the partial order of each input stream is respected).

I must say I'm not sure how this related to `parallel`, or the `merge` evoked in #30, but I thought I'd submit the code I got to work for feedback.