Accept fn for lazy sequential streams

[richardscarrott]

I haven't looked into the implementation of merge2 yet but wondered whether it would be possible to accept a function (() => Stream), allowing streams to be lazy, e.g.

const pipeline1 = () => readable1.pipe(transform1); // Lazy
const pipeline2 = () => readable2.pipe(transform2); // Lazy
merge(pipeline1, pipeline2).pipe(writable); // `readable2` is not read from until `pipeline1` is finished