Extending / Overriding Mappings

Currently only an `ignore` option is accepted, which allows to exempt certain imports from being rewritten, e.g. as used by https://github.com/babel/ember-cli-babel/blob/aa6b45cad241617279d24f96390f9ced3ff0e754/index.js#L610-L630.

It would be awesome, if we could expand this API to allow overriding / amending the standard mapping from `ember-rfc176-data`.

During the implementation of the [`ember-cached-decorator-polyfill`](https://github.com/ember-polyfills/ember-cached-decorator-polyfill), I had to do some crazy contortions (https://github.com/ember-polyfills/ember-cached-decorator-polyfill/pull/12) to add new entries to the mapping. It would be great, if we could improve the infrastructure for future polyfills here. 💕

I guess this will also require some changes to `ember-cli-babel`, but the more significant changes will happen here.

首先追踪当前的 `ignore` 选项和来自 `ember-rfc176-data` 的标准映射是如何处理的。然后检查与 `ember-cli-babel` 的集成；完成的标准应当是，调用方可以通过清晰的 API 覆盖或补充映射，而无需采用针对 `ember-cached-decorator-polyfill` 所描述的那些曲折做法。

由索引模型根据 Issue 内容生成。