[MonologBridge] add LoggerProxy

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| License       | MIT

This allows easy setup for setter injected logger writing to different channels. http://symfony.com/doc/current/reference/dic_tags.html#dic-tags-monolog

Example service:
```yaml
services:
    my_logger.my_channel:
        class: Symfony\Bridge\Monolog\LoggerProxy
        arguments:
            - '@logger'
        tags:
            - { name: 'monolog.logger', channel: 'my_channel' }
```

I'm not exactly sure, whether this is the best way of solving it, but I don't want to clutter constructors just because of this limitation :-)