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.

首先追蹤 Angular CLI 的建置流程，從 index 產生到 hash 計算。確認這些階段之間可以在哪裡執行 hook，然後確認在 hashing 之前執行 post-generation action 能保留最終的 index hash，並解決 issue 中描述的 Angular service-worker mismatch。

由索引模型根據 Issue 內容生成。