feat: ssr getInitialProps add req, res params

[ycjcl868]

Close #2844.

[sorrycc]

res 用不到吧？

[ycjcl868]

可以做一些跳转和错误处理：

import React from 'react'

export default class extends React.Component {
  static async getInitialProps({ res }) {
    if (res) {
       res.statusCode = 404
       res.end('Not found')
       return
   }
  }
}

[coveralls]

Pull Request Test Coverage Report for Build 4975

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 25 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.05%) to 33.613%

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Files with Coverage Reduction	New Missed Lines	%
packages/umi-build-dev/lib/plugins/commands/ui/src/app.js	10	0.0%
packages/umi-build-dev/src/plugins/history.js	15	0.0%

Totals
Change from base Build 4973:	-0.05%
Covered Lines:	1475
Relevant Lines:	4385

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💛 - Coveralls