[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
Yes it might be useful, but everything is useful for someone. The point is,

1. it violates single responsibility
2. is really simply to do in user-land, just `$messages['mydomain']