File loaders

Merged the work of @domu1de and:

- removed `parseScope()`
- added Added `$container->addDefinitionsFromFile()` (configuration is now done through the container to simplify the usage)
- move file loaders into `DI\Definition\FileLoader` because it's a sub-functionality of "definitions"

Left to do:

[ ] Fix integration tests for YAML (It seems like the YAML file is not read properly)
[ ] Add integration tests for XML (just have to write the XML file)
[ ] Move the "$validate" parameter as a config option of the container: should be set globally so that in dev environment it can be true, and in production it can be false
[ ] Documentation for YAML and XML