Ability to access logger instances in the global scope via static calls

The goal of this pull request is have ability to access created Logger instances in all methods, functions, ... Everything you need to know is the logging channel's name.

```
$logger1 = new Monolog\Logger('application');
$logger2 = new Monolog\Logger('api');

function test()
{
    Monolog\Logger::api()->addDebug('Sent to $logger2');
}
```