Allow users to pass in their own compression-stream function

It looks like [FF now accepts brotli compression](https://bugzilla.mozilla.org/show_bug.cgi?id=366559). I was going to take a stab at implementing a native addon that links libbrotli. Does that seem worthwhile?

We could use this: https://www.npmjs.com/package/brotli, which uses an emscriptenized version of brotli.

Does anyone know of any others brotli modules that could be used? Otherwise, I don't mind starting the C++ add on.

I've been taking a look at how the gzip stream is used here, so I think I can make a nice library that implements the same interface.