Allow for custom ConfigureRoutes implementations

Also in FastRoute:

- rename $configuredRoutes and $routeConfiguration to $configureRoutes for consistency
- add processedRoutes() method to expose processed routes for examination, e.g. by a URL dumper
- add configureRoutes() method to allow testing for the custom implementation
- make RouteCollector non-final so it can be extended by consumers, e.g. to auto-add route names