ReadableStream support for Encoding

Human-readable patch to the Encoding Standard to add support for transform streams.

To see the HTML rendered as HTML, go to http://htmlpreview.github.io/?https://github.com/ricea/encoding-streams/blob/master/patch.html

This enables uses like

const response = await fetch(url)
let stringStream = response.body.pipeThough(new TextDecoderStream());

The Bikeshed source for these changes can be found at https://github.com/ricea/encoding/blob/stream-support/encoding.bs.