[1.0][loader] cleanup filesystem loader, simplify logic, add factory.

This PR removes some config params: `formats` and `data_root`. Adds a factory so now we can configure several filesystem loaders.

```yaml
liip_imagine:
  loaders:
    default:
      filesystem: ~
    foo:
      filesystem:
        data_root: %kernel.root_dir%/../web
```

The logic is working, still have to add and fix some tests.