feat: add helmet plugin

[ycjcl868]

Usage: yarn add @umijs/plugin-helmet，考虑内置到 @umijs/preset-react 中，不需要单独再安装

import { Helmet } from 'umi';


export default function() {
  return (
    <>
      <Helmet>
        <title>Title Helmet</title>
      </Helmet>
    </>
  );
}

normal: <title>Title Helmet</title>

if SSR:
=> <title data-rh=\"true\">Title Helmet</title>

[codecov]

Codecov Report

Merging #180 into master will increase coverage by 0.33%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #180      +/-   ##
==========================================
+ Coverage   51.48%   51.81%   +0.33%     
==========================================
  Files          65       66       +1     
  Lines        1447     1457      +10     
  Branches      404      404              
==========================================
+ Hits          745      755      +10     
  Misses        702      702              

Impacted Files	Coverage Δ
packages/preset-react/src/index.ts	0.00% <ø> (ø)
packages/plugin-helmet/src/index.ts	100.00% <100.00%> (ø)

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[sorrycc]

ssr 依赖这个插件才能正常生成 title 等 head 信息？那应该内置在 umi 里啊。

[ycjcl868]

ssr 依赖这个插件才能正常生成 title 等 head 信息？那应该内置在 umi 里啊。

不需要依赖也可以，就是走 title 配置

[sorrycc]

验证下只是开启这个插件，但是不使用，打包会不会包含 helmt 脚本，也就是会不会被 tree-shaking 掉。

[ycjcl868]

验证下只是开启这个插件，但是不使用，打包会不会包含 helmt 脚本，也就是会不会被 tree-shaking 掉。

验证了，这样就不会打包到 umi.js 中了 97e4bf4