Allow to provide an entrypoint URL to the build command

[ogizanagi]

Instead of scanning all the GET routes available in the app without parameters to crawl the pages,
allow to provide a single entrypoint URL from where to start crawling the pages.

symfony console stenope:build --entrypoint-url=/catalogs build/catalogs
symfony console stenope:build --entrypoint-url=/kiosks build/kiosk

This would allow:

• building multiples, distinct sections of a single Symfony app into multiple dedicated static builds
• naturally ignore some of the existing GET routes that are not meant to be included in the static build

Additionnal needs:

• we might need a way to register some additional routes that are not referenced on any page, but should still be included.
  ➜ To solve such a need, we currently can register a listener on the stenope:build command to add such routes, as we do for member signatures pages on our elao website: https://github.com/Elao/elao_/blob/master/src/Stenope/Listener/BuildListener.php
  But we might need a more official way and allow the user to choose if it should be included or not according to the entrypoint. ➜ Simply create a dedicated event where one could access the entrypoint and add routes to the collection?
  One could also simply rely on an env var as a flag when calling the build command.