Build process hook

### Desired functionality
It would be nice to have a hook in the build process, between the index generation and the hash computation, for execute something.

I try to use the Critical lib with my Angular project. First, I build my app, then I use Critical script in order to inline my CSS in the index file : I rewrite my index file. It was messed up with Angular SW, so I submit an issue in Angular's Github (https://github.com/angular/angular/issues/25267).

@gkalpak explains that the issue result from the stored hash mismatch, because of the rewrite of index file.

首先跟踪 Angular CLI 的构建流程，从 index 生成到 hash 计算。确认这两个阶段之间可以在哪里运行 hook，然后确认在 hashing 之前执行 post-generation action 可以保留最终的 index hash，并解决 issue 中描述的 Angular service-worker mismatch。

由索引模型根据 Issue 内容生成。