[Translation] added method to expose collected message

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | no
| Fixed tickets | #2435 
| License       | MIT
| Doc PR        | symfony/symfony-docs#3386
@Tobion Frankly, don't see anything bad of allowing to filter messages at the fetch method. It looks natural for me. Confident, that other developers will find it usefull.

Regarding "`null` vs. `array()`", I'm like 45/55. Your argument
> to get every domain if you pass an empty list.

seems reasonable for me. That's how most UI filters works "empty filter list --> all records". That's why, have implemented it this way first time. But, really don't see a big deal to use `null` for all messages. There's only matter of integrity and consistency, and IMHO `array()` fits more those requirements.