[Routing] Add support for 'priority' option for annotated Routes

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | [sensiolabs/SensioFrameworkExtraBundle#320]
| License       | MIT

If the given route option parameter has a priority key, it will be sorted. The use case is described in the linked ticket. This is a new implementation, previous one (#11747) was proven bad.

If the changes are acceptable, then I will create the changes also for the Doc
Oh, I see what you mean. The original goal was that if you import the routes for the Controller directory, you can set up priority between the files.