message: add baseURL for static assets if served from primer.style
diff: import React from 'react'
  import Document, {Head, Main, NextScript} from 'next/document'
  import {ServerStyleSheet} from 'styled-components'
  +import {alias as aliasHostname} from '../now.json'
  +import {parse} from 'url'
  +
  +const deployURL = process.env.NOW_URL || ''
  +const deployHostname = parse(deployURL).hostname
  export default class MyDocument extends Document {
  - static getInitialProps({renderPage}) {
  + static getInitialProps({renderPage, req}) {
  const sheet = new ServerStyleSheet()
  const page = renderPage(App => props => sheet.collectStyles(<App {...props} />))
  const styles = sheet.getStyleElement()
  + const {hostname} = req.headers
  + const baseURL = deployHostname && (hostname !== deployHostname) ? deployURL : ''
  return {
  ...page,
  - styles
  + styles,
  + baseURL
  }
  }
  render() {
  - const {styles} = this.props
  + const {baseURL, styles} = this.props
  return (
  <html lang="en">
  <Head>
  <title>primer-react</title>
  + <meta charSet="utf8" />
  + <link rel="icon" href={`${baseURL}/assets/favicon.png`} />
  + <link rel="apple-touch-icon" href={`${baseURL}/assets/apple-touch-icon.png`} />
  + <link rel="stylesheet" href={`${baseURL}/dist/css/build.css`} />
  <meta name="viewport" content="width=device-width,initial-scale=1" />
  <meta name="generator" content="Compositor X0" />
  - <link rel="icon" href="/assets/favicon.png" />
  - <link rel="apple-touch-icon" href="/assets/apple-touch-icon.png" />
  <meta name="og:title" content="Primer React" />
  <meta name="description" content="Primer components built with React.js." />
  - <link rel="stylesheet" href="/dist/css/build.css" />
  {styles}
  </Head>
  <body>
  