Feature: replace

[conartist6]

We should support the JS API for replacement so that the engine can be used to modify streams on the fly.

[conartist6]

Actually there's probably an argument to be made for designing an API from scratch, and then perhaps implementing the JS API as an overlay onto it. I think this is one of those things that can be made a bit more idiomatic in the iterable paradigm, probably by just handing control of generation of output over to another generator whose argument is the match.