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.

Start by tracing the Angular CLI build process from index generation through hash computation. Verify where a hook could run between those stages, then confirm that executing a post-generation action before hashing preserves the final index hash and resolves the Angular service-worker mismatch described in the issue.

Written by the indexing model from the issue text.