[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
Please note what the default means: every domain. It seems wrong to use every domain if you pass an empty list. Null stands for no argument, if you pass no argument to domain, it seems logical to get all domains.