[WIP][FrameworkBundle][Command][Routing] Improve extensibility of router related command

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | none
| License       | MIT
| Doc PR        | none

This is a very light and simple patch (I even thought about using the shorter checklist, but it still doesn't fit the description in the doc). Its point is simply to make Router-related commands easier to extend without having to copy-paste them.

Motivation: I am currently developing an app using several routers at once (using a cascading system, please tell me if you think it might be useful to make a PR for this and I shall prepare one, with doc and everything), and it would be nice to be able to debug each of them and not only the main/master one.