[SwiftmailerBridge] Added mailers

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets |
| License       | MIT
| Doc PR        |

Now you can configure several mailers.

Linked to the following PR:
- [ ] https://github.com/symfony/SwiftmailerBundle/pull/34