Add new directory structure handler

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Fixed tickets | #117
| License       | MIT

This is a first move toward https://github.com/symfony/symfony-standard/issues/584
This allows the use of the new directory structure (not by default).