[Custom Engine]: allow to override consolidate
Eventually, engines are not registered on consolidate and if they don't, there is no way to use them with this module. This is the case of for example [Angular Universal](https://github.com/angular/universal) which has its own Engine and works with Express OOTB without using consolidate like [this](https://github.com/angular/universal-starter/blob/master/src/server.ts#L29). It would be good to be able to do the same in Koa.
