[ResourceBundle][Proposal/idea]The configuration directory can be overwrote easily.

Before this PR, you have to define the configuration directory in `BaseExtension::load` all the time. Now, the default value is `/../Resources/config`and you just need to override this attribute if you want to change it (the path to the root of the bundle is calculated too). It is useful if your bundles embed their own configuration (like Sylius) when use this bundle standalone.