[MonologBridge] Allow SwiftMailerHandler to listen to kernel.terminate.event

This PR accompanies symfony/MonologBundle#51. Both are intended to fix symfony/symfony-standard#425.

MonologBundle configures SwiftMailerHandler as a Monolog handler and injects the proper transport service. 

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | 425
| License       | MIT
| Doc PR        | TODO