add formatter to be configurable

add the possibility to configure the formatter for the handler via the config file for example:
```
	'EnliteMonolog' => array(
		'EnliteMonologService' => array(
			// Handlers, it can be service locator alias(string) or config(array)
			'handlers' => array(
				// by config
				'logstash' => array(
					'name' => 'Monolog\Handler\StreamHandler',
					'args' => array(
						'path' => 'myfile.log',
						'level' => \Monolog\Logger::DEBUG,
						'bubble' => true
					),
					'formatter' => array(
						'name' => 'Monolog\Formatter\LogstashFormatter',
						'args' => array(
							'application' => 'My Application',
						),
					),
				),
			),

		),
	),
```

I did not had time to write tests for it.