[WIP] basepath provider (or should we prefix route_basepath automatically)

The route base path provider provides the route base path as defined in `RoutingExtraBundle`.

Currently the path providers provide **full paths** so paths including, for example, the `cmf/routes` path, the `route_basepath` defined in routing extra.

The alternative to this is to use a specified provider

````yaml
base:
  provider: specified
  path: %symfony_cmf_routing_extra.route_basepath%
````

We can also avoid this problem by prefixing this path automaticaly, which would make semantical sense in a way, at the cost of increasing the complexity of the provider code a little.