[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, you are definitely right. It won't be hard to pick proper domains by array's keys in afterwords. I've updated the method, the test and documentation as well.
Thank you.