feat(stylus): 添加stylus支持

[fengxinming]

修改了两个模块 af-webpack 和 umi-build-dev

Checklist

• npm test passes
• tests are included
• documentation is changed or added
• commit message follows commit guidelines

Description of change

• any feature?
• close [Feature Request] 支持stylus-loader #2598

[netlify]

Ant Design Pro preview using the current branch umi

Built with commit 18a9bbd

https://deploy-preview-3483--pro-pre-umijs.netlify.com

[coveralls]

Pull Request Test Coverage Report for Build 8149

• 11 of 16 (68.75%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.1%) to 39.534%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
packages/af-webpack/src/getUserConfig/configs/stylus.js	1	2	50.0%
packages/af-webpack/src/getWebpackConfig/css.js	10	14	71.43%

Totals
Change from base Build 8148:	0.1%
Covered Lines:	4988
Relevant Lines:	12494

---

💛 - Coveralls

[ycjcl868]

这里没必要常量

[fengxinming]

@ycjcl868 我一般写工具都比较灵活，这里主要是配合配合解构，现在的代码合并不了，是这个原因吗？这个工程运行npm run test报错某个文件找不到，目前先build模块代码，把build后的几个文档复制到本地demo中测试正常。

[fengxinming]

@ycjcl868 请问本地如何跑单元测试，如何指定build哪几个模块，能不能在工程中加一个readme？

[sorrycc]

https://github.com/umijs/umi/blob/master/CONTRIBUTING.md#common-tasks
这里有写。

[fengxinming]

@sorrycc 好滴，没问题就尽快合并吧！

[sorrycc]

需要加一个用例。

[fengxinming]

需要加一个用例。

在af-webpack模块下增加了一个stylus的单元测试，yarn debug af-webpack模块正常，现在需要怎么弄？@sorrycc
PS: 那个CONTRIBUTING.md文件需要再写详细点，直接yarn debug都是报错。

[Wxh16144]

请教一下，我正在使用Umi 3.2.3
想在项目中使用 stylus，然后看到这个PR已经合并了
但是文档中又说明内置不支持 sass和 stylus (有点疑惑了)
我应该怎样去看我当前是否支持 stylue，谢谢

[fengxinming]

@Wxh16144 umi3废弃调了之前的webpack配置重新写了，所以这代码不见了

[transtone]

.umirc.js

export default {
  // ... more config
  chainWebpack(config, { createCSSRule }) {
     createCSSRule({
      lang: 'stylus',
      type: 'csr',
      test: /\.styl$/,
      loader: 'stylus-loader',
    });
  },
}