Support for multiple "entries"

Having this implemented would resolve [this issue](https://github.com/developit/unistore/issues/62).

The idea is that we need to output private `package.json` files to support appropriate es/cjs resolving, I believe that @kentcdodds got it "right" [here](https://github.com/kentcdodds/kcd-scripts/blob/d30389cf1c9805556fda59cc4d413763db92808e/src/scripts/build/rollup.js#L68-L85). Although you might prefer having some convention for creating additional "entries" over additional config flag.

I will work on implementing this, hopefully some time soon~

首先阅读链接的 unistore issue，以及所引用路径中的 kcd-scripts Rollup 配置。明确额外条目应使用约定还是配置标志，然后将完成标准定义为输出支持适当 ES 和 CommonJS 解析的私有 package.json 文件。

由索引模型根据 Issue 内容生成。