feat(umi-build-dev): support config via env, e.g. config.{env}.js

[sorrycc]

Notes:

• 如果有配 UMI_ENV，比如 UMI_ENV=cloud，则会用 config/config.cloud.js 合并 config/config.js
• dev 模式下时会额外加载 .local.js，并合并到配置里
• 此方式同样支持 .umirc.js，比如 .umirc.local.js
• *.local.js 的配置文件通常需要添加到 .gitignore，不要提交

[coveralls]

Pull Request Test Coverage Report for Build 458

• 8 of 29 (27.59%) changed or added relevant lines in 2 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.1%) to 21.557%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
packages/umi-build-dev/src/registerBabel.js	0	8	0.0%
packages/umi-build-dev/src/UserConfig.js	8	21	38.1%

Files with Coverage Reduction	New Missed Lines	%
packages/umi-build-dev/src/registerBabel.js	1	0.0%

Totals
Change from base Build 452:	0.1%
Covered Lines:	483
Relevant Lines:	2471

---

💛 - Coveralls

[yutingzhao1991]

这样好像不对，umi dev 的时候需要用 config.local.js assign 到配置，但是依然要保留用户传入的 UMI_ENV。比如 UMI_ENV=cloud umi dev 的情况下配置应该是：

{
  ...require('config.js'),
  ...require('config.cloud.js'),
  ...require('config.local.js'),
}

[sorrycc]

ok，我更新下。

[sorrycc]

Updated @yutingzhao1991

[yutingzhao1991]

是不是少了一个 config.local.js

[sorrycc]

是的，已加。