add umi-plugin-polyfills

[xiaohuoni]

defined import @babel/polyfill setprototypeof
use case:
.umirc.js

export default {
  plugins: [
    "umi-plugin-polyfills"
  ]
};

or

export default {
  plugins: [
    ["umi-plugin-polyfills",{
      extend:['model name']
    }]
  ]
};

extend must be a array ，defined null，
extend:['model name','./src/polyfills.js']
#413

[coveralls]

Pull Request Test Coverage Report for Build 471

• 0 of 32 (0.0%) changed or added relevant lines in 2 files are covered.
• 125 unchanged lines in 3 files lost coverage.
• Overall coverage decreased (-0.3%) to 21.268%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
packages/umi-plugin-polyfill/src/global.js	0	7	0.0%
packages/umi-plugin-polyfill/src/index.js	0	25	0.0%

Files with Coverage Reduction	New Missed Lines	%
packages/af-webpack/src/getConfig.js	4	62.55%
packages/umi-build-dev/src/UserConfig.js	49	19.15%
packages/umi-build-dev/src/Service.js	72	0.0%

Totals
Change from base Build 459:	-0.3%
Covered Lines:	484
Relevant Lines:	2508

---

💛 - Coveralls

[sorrycc]

@babel 依赖在正式发布前需要写死版本，用 7.0.0-beta.46，并且不要用 ^，和其他库的保持一致，统一测试后升级。

[xiaohuoni]

ok

[sorrycc]

name 不对，我觉得叫 umi-plugin-polyfill 比较合适，和 @babel/polyfill 的命名保持一致。

[sorrycc]

extend 是做啥的？import 额外的 js ?

[sorrycc]

现在是否只解决了 IE 10 及以下的兼容问题？

[xiaohuoni]

extend 用于加入其他的兼容文件，支持直接写模块名，支持本地相对应根目录的文件路径。
现在只解决了ie10及以上的兼容问题，未测试ie8、9等浏览器

[sorrycc]

再加个 README.md 介绍下用途吧。

[xiaohuoni]

ok

[codering]

这里只用\n是不是好些？