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.