[Routing] Add createRoute method for AnnotationClassLoader

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | ~
| License       | MIT
| Doc PR        | ~

This allows to use a custom route class. So if you want to support
annotations for Routing in Silex it can now be done while keeping the
custom routing options Silex supports.