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.

Comece rastreando o processo de build do Angular CLI, desde a geração do index até o cálculo do hash. Verifique onde um hook poderia ser executado entre essas etapas e, em seguida, confirme que executar uma ação pós-geração antes do hashing preserva o hash final do index e resolve o Angular service-worker mismatch descrito na issue.

Escrita pelo modelo de indexação a partir do texto da issue.