Add SSR streaming API and stuff

[emmatown]

What:
Add two new exports from emotion-server.

renderStylesToString
This returns a string of html that inlines the critical css required right before it's used.

import { renderToString } from 'react-dom/server'
import { renderStylesToString } from 'emotion-server'
import App from './App'

const html = renderStylesToString(renderToString(<App />))

renderStylesToNodeStream
This returns Node Stream Writable that can be used to insert critical css right before it's required. It can be used with React's streaming API.

import { renderToNodeStream } from 'react-dom/server'
import { renderStylesToNodeStream } from 'emotion-server'
import App from './App'

const stream = renderToNodeStream(<App />).pipe(renderStylesToNodeStream())

I'm not totally sure about the names of these.

There are also some internal changes to extractCritical that should make it faster.

This also fixes some stuff with SSR and labels.

Why:
Closes #341

How:
glamor-stream, https://github.com/threepointone/glamor/blob/master/src/inline.js and some changes in a few places.

Checklist:

• Documentation
• Tests
• Code complete

[tkh44]

Should this just accept an array as the second argument and have the function return an object with the updated keys and tag?

[emmatown]

Yeah, that's probably a better idea.

[tkh44]

Could use this to avoid having to name the arg.

function write([type, data])

[emmatown]

Yup, it was originally like that but I was debugging something so I changed it.