[TwigBundle] Add loader priority

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   |  
| Fixed tickets | 
| License       | MIT
| Doc PR        |  

Add the ability to specify a priority to the tag of `twig.loader` services.

eg:
```
        <service id="twig.loader.filesystem" class="%twig.loader.filesystem.class%" public="false">
            <argument type="service" id="templating.locator" />
            <argument type="service" id="templating.name_parser" />
            <tag name="twig.loader" priority="100"/>
        </service>
```
this can be much faster by using nested arrays instead, as done in other places:

```php
$prioritizedLoaders = array();

foreach ($loaderIds as $id => $tags) {
    foreach($tags as $tag) {
        $priority = isset($attribute['priority']) ? $attribute['priority'] : 0;
        $prioritizedLoaders[$priority][] = $id;
    }
}

ksort($prioritizedLoaders);

foreach ($prioritizedLoaders as $loaders) {
    foreach ($loaders as $loader) {
        $chainLoader->addMethodCall('addLoader', array(new Reference($loader)));
    }
}
```