Add support to xlf without losing xliff support

Since Symfony changed the extension of translation files from `xliff` to `xlf` the bundle is **not** rendering all messages for the domain.

Using `translation.xml` as base for this commit:

    <service id="translation.loader.xliff" class="%translation.loader.xliff.class%">
        <tag name="translation.loader" alias="xlf" legacy-alias="xliff" />
    </service>

I've updated the `AddLoaderPass.php` to use the attributes available instead of guessing the extension.

Related to #36