Update readme since `WritableStream`s seem supported now

[dario-piotrowicz]

Description

Hey @DylanPiercey just wanted to say that I think that WritableStreams are now supported across the board (by firefox since May according to caniuse).

So I guess the Readme could be updated to present only the shorter syntax 🙂

And the code could actually be cleaned up to use directly a class instead of a constructor function I'd imagine?

If you're interested let me know as I'd gladly make the changes 😄

[DylanPiercey]

I think updating the readme would be good. However I'd still like to show both examples and keep the underlying impl as is for best browser support.

[dario-piotrowicz]

Ok, just updating the readme then! I gave it a shot please have a look 🙂